Fix false positive "offset might not exist" for string offset in union

- Skip "might not exist" report for string types with integer offsets in
  NonexistentOffsetInArrayDimFetchCheck decomposition check
- When a union of constant strings (e.g. ''|':') is accessed with a valid
  integer offset, the decomposition incorrectly flagged it because one
  member (empty string) returned "no" for the offset
- The union-level check already correctly returns "maybe" for such cases
- New regression test in tests/PHPStan/Rules/Arrays/data/bug-13688.php

Closes phpstan/phpstan#13688

Author: github-actions[bot]

src/Rules/Arrays/NonexistentOffsetInArrayDimFetchCheck.php

@@ -126,6 +126,9 @@ public function check(
 					if (
 						$innerType->hasOffsetValueType($innerDimType)->no()
 					) {
+						if ($innerType->isString()->yes() && $innerDimType->isInteger()->yes()) {
+							continue;
+						}
 						$report = true;
 						break 2;
 					}

tests/PHPStan/Rules/Arrays/NonexistentOffsetInArrayDimFetchRuleTest.php

@@ -1277,4 +1277,9 @@ public function testBug14308(): void
 		$this->analyse([__DIR__ . '/data/bug-14308.php'], []);
 	}
 
+	public function testBug13688(): void
+	{
+		$this->analyse([__DIR__ . '/data/bug-13688.php'], []);
+	}
+
 }

tests/PHPStan/Rules/Arrays/data/bug-13688.php

@@ -0,0 +1,12 @@
+<?php declare(strict_types = 1);
+
+namespace Bug13688;
+
+$inputs = [ '', ':' ];
+
+foreach ( $inputs as $input )
+{
+	$inputLen = \strlen($input);
+	$hasTrailingColon = $inputLen > 0 && $input[$inputLen-1] === ':';
+	echo $hasTrailingColon ? "{$input} has trailing colon\n" : "{$input} does not have trailing colon\n";
+}