Update input descriptions

We want to make thse messages consistent and read in the positive.
"not empty" is harder to understand than "provided".

Also, let's match the description formats.

action.yml

@@ -13,11 +13,11 @@ inputs:
     required: false
     default: true
   delete_prev_regex_msg:
-    description: If not empty, all messages matching the given regex will be deleted.
+    description: If provided, all messages matching the given regex will be deleted.
     required: false
     default: ~
   duplicate_msg_pattern:
-    description: Optional pattern to use when checking for duplicate message.
+    description: If provided, the regex pattern will be used when checking for duplicate messages.
     required: false
     default: ~