fix(web): crash on custom modifier keys

Fixes #6788.
Fixes KEYMAN-WEB-22.
Fixes KEYMAN-DEVELOPER-BQ.
Fixes KEYMAN-ANDROID-160.

Fixes a crash introduced in #6473 which was trying to prevent modifier
keys from triggering "fat finger" alternate lookups, due to incomplete
nullish coalescing.

Related to this, from what I can see, a secondary side-effect of the fix
in #6473 was that some key events may have never had their ruleBehaviors
finalized if they matched the `isOnlyLayerSwitchKey` heuristic, leading
to potential issues with edge case 'deadkey+layer switch' keys or keys
that set store values, for example. So this fix also makes the
`isOnlyLayerSwitchKey` test more targeted.

Identified this when adding a Caps Lock layer to sil_euro_latin and
testing the layer switching. At time of fix, the error had been raised
in KeymanWeb, Keyman Developer and Keyman for Android, but not yet
Keyman for iPhone and iPad.