App-Search vs Ent-Search difference in filter

Hello,
I'm a newbie in those topics, and really need to understand why the behavior in App-Search and Ent-Search is different in the case of empty string used in the filter.
Example:
When I use the filter in the query:
"query": "example",
"filters" : {
"all": [
{ "manid": "" },
{ "catid": "" }
]
}

The app-Search engine is returning all results containing search string "example" and does not matter what is stored in manid and catid, even keyword ALL indicates AND. The empty string is accepted as a wildcard.

But in Enterprise-Search, the same (migrated) engine, returns nothing! When I replace empty string with appropriate values, then return proper results.

Now a good question, WHY? And how we can fix it? Because that's really blocking upgrading App-Search to new Enterprise-Search.

Can somebody help me?

Thank you in advance
Best Regards
Rudolf