Fix scope namespace not reset when entering global namespace block

- In NodeScopeResolver, when processing a `namespace { }` block (global namespace)
  after a named namespace block, the scope's namespace was not being reset to null
- This caused PHPDoc resolution to use the wrong function name (prefixed with the
  previous namespace), so PHPDoc annotations like `@return list<string>` were lost
- Added `else` branch in namespace handling to call `enterNamespace('')` for global
  namespace blocks, which MutatingScope normalizes to `null`
- New regression test in tests/PHPStan/Rules/Functions/data/bug-13831.php

Closes phpstan/phpstan#13831

Author: github-actions[bot]

src/Analyser/NodeScopeResolver.php

@@ -1010,6 +1010,8 @@ private function processStmtNode(
 		} elseif ($stmt instanceof Node\Stmt\Namespace_) {
 			if ($stmt->name !== null) {
 				$scope = $scope->enterNamespace($stmt->name->toString());
+			} else {
+				$scope = $scope->enterNamespace('');
 			}
 
 			$scope = $this->processStmtNodesInternal($stmt, $stmt->stmts, $scope, $storage, $nodeCallback, $context)->getScope();

tests/PHPStan/Rules/Functions/MissingFunctionReturnTypehintRuleTest.php

@@ -82,4 +82,9 @@ public function testRule(): void
 		]);
 	}
 
+	public function testBug13831(): void
+	{
+		$this->analyse([__DIR__ . '/data/bug-13831.php'], []);
+	}
+
 }

tests/PHPStan/Rules/Functions/data/bug-13831.php

@@ -0,0 +1,14 @@
+<?php
+
+namespace Bug13831 {
+	// non-global namespace block
+}
+
+namespace {
+
+	/** @return list<string> */
+	function bug13831Qux(): array {
+		return [ random_bytes(16) ];
+	}
+
+}