Fix the nonempty template fixture used by CI

Return the fixture input so its implementation satisfies the nonempty return type inferred from its template bound.

Document clearing the PHPStan result cache before verification. Cached results had hidden the return-type error in the unchanged fixture during local checks.

Author: jbboehr

docs/development/behavior-contract.md

@@ -58,13 +58,17 @@ does not mandate one internal type representation for those combinations.
 ```sh
 composer phpunit -- --no-coverage --filter 'ArrayMergeContractTest|ArrayMergeNativeBehaviorTest'
 composer phpunit -- --no-coverage
+composer phpstan -- clear-result-cache
 composer phpstan -- analyze --no-progress
 composer phpcs
 ```
 
 Use PHPUnit's `--filter` with a dataset name to replay one combination. CI also
 runs these tests in the existing PHPStan compatibility jobs.
 
+Clear PHPStan's result cache when checking changes to the extension. Cached
+results can hide new diagnostics in fixtures whose source has not changed.
+
 When a test fails, check fixture validity and the independent expected behavior
 before changing either production code or an assertion. Keep a minimal named
 case for a demonstrated defect. Do not replace expectations with the current

tests/fixtures/functions.php

@@ -79,7 +79,7 @@ function nested(array $a, array $b, array $c): array
  */
 function genericMergeUnionOperand(array $value): array
 {
-    return [];
+    return $value;
 }
 
 /**