fix: Functions\ReturnTypeSniff for mixed declaration when returning null (#186)

Closes #185

Author: michalbundyra

src/WebimpressCodingStandard/Sniffs/Functions/ReturnTypeSniff.php

@@ -974,10 +974,10 @@ private function getReturnValue(File $phpcsFile, int $ptr) : string
                 return 'new';
 
             case T_NULL:
-                if (! $this->hasCorrectType([], ['null'])
-                    || ($this->returnType
-                        && $this->returnTypeIsValid
-                        && strpos($this->returnTypeValue, '?') !== 0)
+                if (! $this->hasCorrectType(['null', 'mixed'], ['null', 'mixed'])
+                    && (! $this->returnType
+                        || ($this->returnTypeIsValid
+                            && strpos($this->returnTypeValue, '?') !== 0))
                 ) {
                     $error = 'Function return type is not nullable, but function returns null here';
                     $phpcsFile->addError($error, $ptr, 'ReturnNull');

test/Sniffs/Functions/ReturnTypeUnitTest.inc

@@ -406,4 +406,17 @@ abstract class FunctionCommentReturn
 
         return new mysqli_result;
     }
+
+    public function getStr(string $key): mixed
+    {
+        if (is_string($key)) {
+            return $key;
+        }
+
+        if (is_array($key)) {
+            return json_encode($key);
+        }
+
+        return null;
+    }
 }