fix the bug as discussed on Pull request #7

Author: sylikc

exiftool/exiftool.py

@@ -563,7 +563,7 @@ def get_metadata(self, filename, params=None):
 		documentation of :py:meth:`execute_json()`.
 		"""
 		if not params:
-			params = None
+			params = []
 		return self.execute_json(filename, *params)[0]
 
 	# (#11)