Bring back nil check for constructing credential error msg

Author: cacheflow

lib/fcm.rb

@@ -295,9 +295,14 @@ def jwt_token
 
   def credentials_error_msg(param)
     error_msg = 'credentials must be an IO-like ' \
-      'object or path You passed '
+      'object or path '
+    
+    if param.nil?
+      error_msg += 'You passed nil'
+    else 
+      error_msg += "You passed a #{param.class.name}"
+    end
 
-    error_msg += param.class.name.to_s
     raise InvalidCredentialError, error_msg
   end