Shows 'No result found' if custom field is blank

You should have to enter the Search All text, otherwise its treated as no value inserted.

Do you mean I need to add “Search All” (or whatever text I use in the “Search All” field) in custom post’s blank custom fields?

If so then I have a problem. Some custom posts can’t have some custom fields, according to the requirement.

So isn’t it a good feature to add in the plugin which will consider the “Search All” text for the blank custom fields?

Thanks.

This is how “Search All” wokrs:
When user using the ‘Search All’ text, it will bypass that filter, which mean if you have a single custom field filter whit Search All option in the search form, when you search for “Search All” , the search will bypass this field and only filter the posts with the custom post types that you configured in the form setting page.

As for your problem,

custom posts can’t have some custom fields

If there are no custom fields attached to the posts, the posts will not be searched with that filter. But if you using the Search All to search, then the search will fall back to the custom post type. Which mean all of your posts in the custom post type will be search regardless the custom fields.