[Php81] Skip array dim fetch args in NullToStrictStringFuncCallArgRector

[TomasVotruba]

NullToStrictStringFuncCallArgRector now skips array dim fetch args ($array['key']).

Array dim fetch values are mixed — the null is not known, so casting to (string) may be wrong. Now explicitly skipped:

 public function run(array $data)
 {
-    return str_replace('a', 'b', (string) $data['key']);
+    return str_replace('a', 'b', $data['key']);
 }