Fix nullrefs when making operations

Author: desplesda

YarnSpinner/QuestGraphs/QuestGraph.Operations.cs

@@ -58,7 +58,7 @@ public static Expression MakeAnd(IEnumerable<Expression?> children)
 
             var result = new Expression
             {
-                And = { }
+                And = new NAryExpression()
             };
             result.And.Children.AddRange(expressions);
             return result;
@@ -94,9 +94,9 @@ public static Expression MakeOr(IEnumerable<Expression?> children)
 
             var result = new Expression
             {
-                Or = { }
+                Or = new NAryExpression()
             };
-            result.And.Children.AddRange(expressions);
+            result.Or.Children.AddRange(expressions);
             return result;
         }
 
@@ -114,7 +114,7 @@ public static Expression MakeNot(Expression? child)
 
             return new Expression
             {
-                Not = { Expr = child },
+                Not = new UnaryExpression { Expr = child },
             };
         }