Try to prevent findGitHash from ever returning None

overviewer_core/util.py

@@ -43,6 +43,7 @@ def findGitHash():
         line = p.stdout.readlines()[0].strip()
         if line and len(line) == 40 and all(c in hexdigits for c in line):
             return line
+        return "unknown"
     except Exception:
         try:
             import overviewer_version