Restore null default for globalBasename

It was changed in 3e2fad5, I am not
sure why, I think it might be to avoid the extra conditions / complexity
I'm adding here. What I know is that it makes a guard condition in
FileDriver::initialize() useless, most likely resulting in persistence
looking for a file with a name consisting only in an extension (.php,
.yml…).

Author: greg0ire

src/Persistence/Mapping/Driver/FileDriver.php

@@ -33,8 +33,8 @@ abstract class FileDriver implements MappingDriver
      */
     protected $classCache;
 
-    /** @var string */
-    protected $globalBasename = '';
+    /** @var string|null */
+    protected $globalBasename = null;
 
     /**
      * Initializes a new FileDriver that looks in the given path(s) for mapping
@@ -132,7 +132,7 @@ public function getAllClassNames()
             $this->initialize();
         }
 
-        if ($this->classCache === []) {
+        if ($this->classCache === [] && $this->globalBasename !== null) {
             return $this->locator->getAllClassNames($this->globalBasename);
         }
 
@@ -144,7 +144,7 @@ public function getAllClassNames()
 
         return array_values(array_unique(array_merge(
             $keys,
-            $this->locator->getAllClassNames($this->globalBasename)
+            $this->globalBasename === null ? [] : $this->locator->getAllClassNames($this->globalBasename)
         )));
     }