clarify null behavior of if

Author: robrichard

spec/Section 3 -- Type System.md

@@ -2204,7 +2204,7 @@ fragment someFragment on User {
 
 - `if: Boolean! = true` - When `true`, fragment _should_ be deferred. When
   `false`, fragment will not be deferred and data will be included in the
-  initial response. If omitted, defaults to `true`.
+  initial response. Defaults to `true` when omitted or null.
 - `label: String` - May be used by GraphQL clients to identify the data from
   responses and associate it with the corresponding defer directive. If
   provided, the GraphQL Server must add it to the corresponding payload. `label`
@@ -2240,7 +2240,7 @@ query myQuery($shouldStream: Boolean) {
 
 - `if: Boolean! = true` - When `true`, field _should_ be streamed. When `false`,
   the field will not be streamed and all list items will be included in the
-  initial response. If omitted, defaults to `true`.
+  initial response. Defaults to `true` when omitted or null.
 - `label: String` - May be used by GraphQL clients to identify the data from
   responses and associate it with the corresponding stream directive. If
   provided, the GraphQL Server must add it to the corresponding payload. `label`

spec/Section 6 -- Execution.md

@@ -627,8 +627,8 @@ asyncRecord, visitedFragments, deferredGroupedFieldsList):
   - If {selection} is a {FragmentSpread}:
     - Let {fragmentSpreadName} be the name of {selection}.
     - If {fragmentSpreadName} provides the directive `@defer` and its {if}
-      argument is {true} or is a variable in {variableValues} with the value
-      {true}:
+      argument is not {false} and is not a variable in {variableValues} with the
+      value {false}:
       - Let {deferDirective} be that directive.
     - If {deferDirective} is not defined:
       - If {fragmentSpreadName} is in {visitedFragments}, continue with the next
@@ -668,19 +668,20 @@ asyncRecord, visitedFragments, deferredGroupedFieldsList):
       fragmentType)} is false, continue with the next {selection} in
       {selectionSet}.
     - Let {fragmentSelectionSet} be the top-level selection set of {selection}.
-    - If {InlineFragment} provides the directive `@defer`, let {deferDirective}
-      be that directive.
-      - If {deferDirective}'s {if} argument is {true} or is a variable in
-        {variableValues} with the value {true}:
-        - Let {label} be the value or the variable to {deferDirective}'s {label}
-          argument.
-        - Let {deferredGroupedFields} be the result of calling
-          {CollectFields(objectType, objectValue, fragmentSelectionSet,
-          variableValues, path, asyncRecord, visitedFragments,
-          deferredGroupedFieldsList)}.
-        - Append a record containing {label} and {deferredGroupedFields} to
-          {deferredGroupedFieldsList}.
-        - Continue with the next {selection} in {selectionSet}.
+    - If {InlineFragment} provides the directive `@defer` and its {if} argument
+      is not {false} and is not a variable in {variableValues} with the value
+      {false}:
+      - Let {deferDirective} be that directive.
+    - If {deferDirective} is defined:
+      - Let {label} be the value or the variable to {deferDirective}'s {label}
+        argument.
+      - Let {deferredGroupedFields} be the result of calling
+        {CollectFields(objectType, objectValue, fragmentSelectionSet,
+        variableValues, path, asyncRecord, visitedFragments,
+        deferredGroupedFieldsList)}.
+      - Append a record containing {label} and {deferredGroupedFields} to
+        {deferredGroupedFieldsList}.
+      - Continue with the next {selection} in {selectionSet}.
     - Let {fragmentGroupedFieldSet} be the result of calling
       {CollectFields(objectType, objectValue, fragmentSelectionSet,
       variableValues, path, asyncRecord, visitedFragments,
@@ -945,9 +946,9 @@ subsequentPayloads, asyncRecord):
   - If {result} is an iterator:
     - Let {field} be the first entry in {fields}.
     - Let {innerType} be the inner type of {fieldType}.
-    - If {field} provides the directive `@stream` and its {if} argument is
-      {true} or is a variable in {variableValues} with the value {true} and
-      {innerType} is the outermost return type of the list type defined for
+    - If {field} provides the directive `@stream` and its {if} argument is not
+      {false} and is not a variable in {variableValues} with the value {false}
+      and {innerType} is the outermost return type of the list type defined for
       {field}:
       - Let {streamDirective} be that directive.
     - Let {initialCount} be the value or variable provided to