Skip empty variables in detect_aws_credentials

pre_commit_hooks/detect_aws_credentials.py

diff --git a/pre_commit_hooks/detect_aws_credentials.py b/pre_commit_hooks/detect_aws_credentials.py
index fe18f4d..aae0734 100644
--- a/pre_commit_hooks/detect_aws_credentials.py
+++ b/pre_commit_hooks/detect_aws_credentials.py
@@ -84,7 +84,7 @@
             for key in keys:
                 # naively match the entire file, low chance of incorrect
                 # collision
-                if key in text_body:
+                if key and key in text_body:
                     bad_files.append(BadFile(filename, key[:4].ljust(28, '*')))
     return bad_files