Fixed a invalid offset notice that can be occuring in GitVersionInfo.

src/Services/GitVersionInfo.php

@@ -64,6 +64,9 @@ class GitVersionInfo
             $git = file($this->project_dir.'/.git/HEAD');
             $head = explode('/', $git[0], 3);
 
+            if (!isset($head[2])) {
+                return null;
+            }
             return trim($head[2]);
         }
 
@@ -86,6 +89,10 @@ class GitVersionInfo
             $head = file($filename);
             $hash = $head[0];
 
+            if (!isset($head[0])) {
+                return null;
+            }
+
             return substr($hash, 0, $length);
         }