Fix fieldName in FieldAttribute (elastic#129427) (elastic#129430)

I believe we missed the if/else here in elastic#128910; without it, we won't 
maintain backward compatibility with pre-8.15. This is no longer
necessary in main since it doesn't interact with pre-8.15 nodes. I'll
open a follow-up to remove it from main, but it's required in 8.19 and
8.18 because mixed cluster tests are failing without it.

Relates elastic#128910
Closes elastic#129412
Closes elastic#129411
Closes elastic#129375
Closes elastic#129374
Closes elastic#129373
Closes elastic#129340
Closes elastic#129339
Closes elastic#129304
Closes elastic#129303

Closes elastic#129410
Closes elastic#129409
Closes elastic#129408
Closes elastic#129407
Closes elastic#129379
Closes elastic#129378
Closes elastic#129377
Closes elastic#129376
Closes elastic#129307
Closes elastic#129306
Closes elastic#129305

Author: dnhatn

x-pack/plugin/esql-core/src/main/java/org/elasticsearch/xpack/esql/core/expression/FieldAttribute.java

@@ -204,8 +204,9 @@ public FieldName fieldName() {
             // name starting with `$$`.
             if ((synthetic() || name().startsWith(SYNTHETIC_ATTRIBUTE_NAME_PREFIX)) == false) {
                 lazyFieldName = new FieldName(name());
+            } else {
+                lazyFieldName = new FieldName(Strings.hasText(parentName) ? parentName + "." + field.getName() : field.getName());
             }
-            lazyFieldName = new FieldName(Strings.hasText(parentName) ? parentName + "." + field.getName() : field.getName());
         }
         return lazyFieldName;
     }