Use full path on working direcor and fix the detection of external path

Author: na2axl

src/FireFS/FireFS.php

@@ -252,7 +252,7 @@ public function explodePath(string $path): array
      */
     public function setWorkingDir(string $workingDir)
     {
-        $this->workingDir = $workingDir;
+        $this->workingDir = $this->toExternalPath($workingDir);
     }
 
     /**
@@ -856,6 +856,10 @@ public function toExternalPath(string $internalPath): string
             }
         }
 
+        if (strlen($externalPath) === 0) {
+            return $this->cleanPath("./");
+        }
+
         if ($externalPath[0] != '/') {
             return $internalPath;
         }