Backport to 26.1: Fix nullable bound on DataFetcherResult.newResult - #4443

Backport of #4418 to the 26.x maintenance branch.

Summary:

• Adds the missing nullable generic bound to the zero-argument DataFetcherResult.newResult() factory method.
• Allows Kotlin callers under JSpecify null-marked semantics to use nullable result types.
• Adds a regression source for Kotlin nullable type interoperability.

Problem:

• DataFetcherResult and its Builder allow nullable type parameters.
• The zero-argument newResult() method declared an unbounded T.
• Kotlin therefore interpreted the factory method type parameter as non-null and rejected nullable types.

Testing:

• ./gradlew compileTestKotlin failed before the Java change with Type argument is not within its bounds for DataFetcherResult.newResult<String?>().
• ./gradlew compileTestKotlin passed after the change.
• ./gradlew test --tests graphql.execution.DataFetcherResultTest passed: 26 tests.
• ./gradlew check -x test -x testng passed.
• ./gradlew javadoc passed.
• ./gradlew testng passed: 190 tests.
• ./gradlew test was run locally on Windows and failed in unrelated graphql.parser.MultiSourceReaderTest.can combine files; the failure reproduces when that test is run alone and appears tied to local checkout line endings for src/test/resources/multisource/a.txt (w/crlf).

Issue:
Fixes #4364