Fix new phpstan errors

phpstan-baseline.neon

@@ -285,11 +285,6 @@ parameters:
 			count: 1
 			path: src/Serializer/AbstractSerializer.php
 
-		-
-			message: "#^Cannot cast mixed to string\\.$#"
-			count: 1
-			path: src/Serializer/AbstractSerializer.php
-
 		-
 			message: "#^Call to method getResponse\\(\\) on an unknown class GuzzleHttp\\\\Exception\\\\RequestException\\.$#"
 			count: 1

psalm-baseline.xml

@@ -66,6 +66,11 @@
       <code>SentryProfile|null</code>
     </MoreSpecificReturnType>
   </file>
+  <file src="src/Serializer/AbstractSerializer.php">
+    <RedundantCast occurrences="1">
+      <code>(string) $value</code>
+    </RedundantCast>
+  </file>
   <file src="src/Serializer/RepresentationSerializer.php">
     <InvalidReturnStatement occurrences="1">
       <code>$value</code>

src/Serializer/AbstractSerializer.php

@@ -212,24 +212,22 @@ protected function serializeObject($object, int $_depth = 0, array $hashes = [])
     /**
      * Serializes the given value to a string.
      *
-     * @param mixed $value The value to serialize
+     * @param string $value The value to serialize
      */
-    protected function serializeString($value): string
+    protected function serializeString(string $value): string
     {
-        $value = (string) $value;
-
         // we always guarantee this is coerced, even if we can't detect encoding
         if ($currentEncoding = mb_detect_encoding($value, $this->mbDetectOrder)) {
-            $value = mb_convert_encoding($value, 'UTF-8', $currentEncoding);
+            $encoded = mb_convert_encoding($value, 'UTF-8', $currentEncoding) ?: '<encoding error>';
         } else {
-            $value = mb_convert_encoding($value, 'UTF-8');
+            $encoded = mb_convert_encoding($value, 'UTF-8') ?: '<encoding error>';
         }
 
-        if (mb_strlen($value) > $this->options->getMaxValueLength()) {
-            $value = mb_substr($value, 0, $this->options->getMaxValueLength() - 10, 'UTF-8') . ' {clipped}';
+        if (mb_strlen($encoded) > $this->options->getMaxValueLength()) {
+            $encoded = mb_substr($encoded, 0, $this->options->getMaxValueLength() - 10, 'UTF-8') . ' {clipped}';
         }
 
-        return $value;
+        return $encoded;
     }
 
     /**
@@ -276,7 +274,11 @@ protected function serializeValue($value)
             return 'Array of length ' . \count($value);
         }
 
-        return $this->serializeString($value);
+        if (\is_string($value) || (\is_object($value) && method_exists($value, '__toString'))) {
+            return $this->serializeString((string) $value);
+        }
+
+        return null;
     }
 
     private function formatDate(\DateTimeInterface $date): string