Search/ filter for "not empty" (custom) fields

How can our customer search for "not empty" (custom) fields without searching a particular search term/ phrase?
The content/ value of the field is irrelevant - we want to search for issues where a particular field is filled in at all.

Our customer's main problem is the following:
How can we filter all Standard Issue Types (or Sub-Task Issue Types), which already have an Original Estimate?

But there are other scenarios (with custom fields involved) where we miss the possibility of searching for fields "that are not empty"
(a) find all issues where there is any "Fixed in Build" (Text Field (< 255 characters) with Free Text Searcher)
(b) find all issues which already have a "Release Note" (Free Text Field (unlimited text) with Free Text Searcher)