statusd.py: certs_readable() checks if cert is defined

statusd.py

@@ -42,9 +42,13 @@ def read_config(CONFIGFILE, CONFIG):
 
 
 def certs_readable(config):
-
+    '''
+    checks at start, if the needed certificates defined (no nullstring) and readable.
+    param 1: dictionary
+    return: boolean
+    '''
     for i in (config['SERVER_KEY'], config['SERVER_CERT'], config['CLIENT_CERT']):
-        if os.access(i, os.R_OK) == False:
+        if  i == '' or os.access(i, os.R_OK) == False:
             logging.error('Cant read {}'.format(i))
             return False
     return True