Add tests

Author: VincentLanglet

tests/PHPStan/Analyser/nsrt/bug-12312.php

@@ -14,6 +14,9 @@ public function sayLowercase(string $s): void
 if ($s != '') {
 assertType('lowercase-string&non-empty-string', $s);
 }
+if ('' != $s) {
+assertType('lowercase-string&non-empty-string', $s);
+}
 assertType('lowercase-string', $s);
 }
 
@@ -25,6 +28,9 @@ public function sayUppercase(string $s): void
 if ($s != '') {
 assertType('non-empty-string&uppercase-string', $s);
 }
+if ('' != $s) {
+assertType('non-empty-string&uppercase-string', $s);
+}
 assertType('uppercase-string', $s);
 }
 
@@ -36,6 +42,9 @@ public function sayBoth(string $s): void
 if ($s != '') {
 assertType('lowercase-string&non-empty-string&uppercase-string', $s);
 }
+if ('' != $s) {
+assertType('lowercase-string&non-empty-string&uppercase-string', $s);
+}
 assertType('lowercase-string&uppercase-string', $s);
 }
 }