[ur] Match null DDI table entry error code

When validation is disabled and an adapter does not sets a DDI table
entry to null the loader returns `UR_RESULT_ERROR_UNINITIALIZED` however
when validation is enabled `UR_RESULT_ERROR_FEATURE_UNSUPPORTED` is
returned. This patch aligns the error codes return by the loader and
validation layer when a null DDI table entry is found.

Author: kbenzie

scripts/templates/valddi.cpp.mako

@@ -47,8 +47,9 @@ namespace ur_validation_layer
     {
         auto ${th.make_pfn_name(n, tags, obj)} = context.${n}DdiTable.${th.get_table_name(n, tags, obj)}.${th.make_pfn_name(n, tags, obj)};
 
-        if( nullptr == ${th.make_pfn_name(n, tags, obj)} )
-            return ${X}_RESULT_ERROR_UNSUPPORTED_FEATURE;
+        if( nullptr == ${th.make_pfn_name(n, tags, obj)} ) {
+            return ${X}_RESULT_ERROR_UNINITIALIZED;
+        }
 
         if( context.enableParameterValidation )
         {