Translate comment into code and annotations

The phpdoc comment for the return type of
ClassMetadata::fullyQualifiedClassName() says that the return type will
be null if the input value is null. I have made it more precise by
using "if and only if", made the null check more strict and translated
that into template annotations. Also, since we say we return a
class-string, I've asserted that.

phpstan-baseline.neon

@@ -125,11 +125,6 @@ parameters:
 			count: 1
 			path: src/EntityRepository.php
 
-		-
-			message: "#^Method Doctrine\\\\ORM\\\\Mapping\\\\ClassMetadata\\:\\:fullyQualifiedClassName\\(\\) should return class\\-string\\|null but returns string\\|null\\.$#"
-			count: 1
-			path: src/Mapping/ClassMetadata.php
-
 		-
 			message: "#^If condition is always true\\.$#"
 			count: 1

psalm-baseline.xml

@@ -290,11 +290,6 @@
       <code>$repositoryClassName</code>
     </ArgumentTypeCoercion>
   </file>
-  <file src="src/Mapping/Builder/EntityListenerBuilder.php">
-    <PossiblyNullArgument>
-      <code>$class</code>
-    </PossiblyNullArgument>
-  </file>
   <file src="src/Mapping/ClassMetadata.php">
     <DeprecatedProperty>
       <code><![CDATA[$this->columnNames]]></code>
@@ -314,25 +309,14 @@
     </InvalidNullableReturnType>
     <InvalidPropertyAssignmentValue>
       <code>$definition</code>
-      <code><![CDATA[$this->subClasses]]></code>
     </InvalidPropertyAssignmentValue>
-    <LessSpecificReturnStatement>
-      <code>$className</code>
-      <code>$className</code>
-      <code><![CDATA[$this->namespace . '\\' . $className]]></code>
-    </LessSpecificReturnStatement>
-    <MoreSpecificReturnType>
-      <code>class-string|null</code>
-    </MoreSpecificReturnType>
     <NullableReturnStatement>
       <code><![CDATA[$this->reflClass]]></code>
     </NullableReturnStatement>
     <ParamNameMismatch>
       <code>$entity</code>
     </ParamNameMismatch>
     <PossiblyNullArgument>
-      <code>$class</code>
-      <code>$className</code>
       <code><![CDATA[$mapping['targetEntity']]]></code>
       <code><![CDATA[$mapping['targetEntity']]]></code>
       <code><![CDATA[$parentReflFields[$embeddedClass->declaredField]]]></code>

src/Mapping/ClassMetadata.php

@@ -2465,12 +2465,16 @@ public function getAssociationMappedByTargetField(string $assocName): string
     }
 
     /**
-     * @return string|null null if the input value is null
-     * @psalm-return class-string|null
+     * @param C $className
+     *
+     * @return string|null null if and only if the input value is null
+     * @psalm-return (C is string ? string : null)
+     *
+     * @template C of string|null
      */
     public function fullyQualifiedClassName(string|null $className): string|null
     {
-        if (empty($className)) {
+        if ($className === null) {
             return $className;
         }
 
@@ -2514,12 +2518,8 @@ public function mapEmbedded(array $mapping): void
             throw MappingException::missingEmbeddedClass($mapping['fieldName']);
         }
 
-        $fqcn = $this->fullyQualifiedClassName($mapping['class']);
-
-        assert($fqcn !== null);
-
         $this->embeddedClasses[$mapping['fieldName']] = EmbeddedClassMapping::fromMappingArray([
-            'class' => $fqcn,
+            'class' => $this->fullyQualifiedClassName($mapping['class']),
             'columnPrefix' => $mapping['columnPrefix'] ?? null,
             'declaredField' => $mapping['declaredField'] ?? null,
             'originalField' => $mapping['originalField'] ?? null,