Fix opening brushes created during incomplete lightness feature implementation

This is a bugfix for https://bugs.kde.org/show_bug.cgi?id=431937

Test Plan

1. Test custom brushes created
   • before lightness mask feature
   • after lightnessmask feature, but before gradientmask feature
   • modern RGBA brushes
2. All the brushes should work correctly and automatically select mode alpha-mask, lightness or gradient

Testing packages

WARNING: The packages are against master branch, please backup your settings before testing!

• (dk1) Windows 64: https://yadi.sk/d/t2erIn2TVCGAgQ
• (dk1) AppImage: https://yadi.sk/d/5k_yu22erNuNEg
• (dk2) Windows 64: https://yadi.sk/d/FSEGJF2M5EFIWw
• (dk2) AppImage: https://yadi.sk/d/l7Pg7zoWjxyVag

Technical details

We had a period of time, when the presence or absence of color in the brush (not transparency) was used to decide whether to use useColorAsAlpha. Basically, absence of color in the brush would switch mask mode even when useColorAsAlpha was false.

In the new versions of Krita 'useColorAsAlpha' tag is completely removed, and 'brushApplication' tag is used instead, which does allow such ambiguity.

BUG:431937