[Highlighters]: forbid to save blank patterns or count them as "nothing" instead of "everything"

[PetbkA]

OS: Windows 10 version 22H2 x64 App Version: 23.07.0.1541 x64 Qt6

Steps to reproduce:

1. Open Highlighters → Configure highlighters...;
2. Create new highlighter with empty Matching Pattern and set e.g. pink background color for it;
3. Activate highlighter set with such highlighter - NOK, all lines are being highlighted with color from step 2 'cause empty pattern is being counted as "everything".

I suggest to either forbid changing highlighter in editor until its pattern will not be empty or count empty patterns as "nothing" instead of "everything"

[variar]

Indeed empty pattern should mean "nothing" in this case