bug(web): empty-string style parse lacks proper default

[sentry-io]

Sentry Issue: KEYMANWEB-COM-1HG

TypeError: null is not an object (evaluating 'e.val')
  at new ParsedLengthStyle (https://s.keyman.com/kmw/engine/17.0.308/src/web/src/engine/osk/src/lengthStyle.ts:20:16)
  at _VisualKeyboard.constructLayoutParams (https://s.keyman.com/kmw/engine/17.0.308/src/web/src/engine/osk/src/visualKeyboard.ts:1305:27)
  at _VisualKeyboard.refreshLayout (https://s.keyman.com/kmw/engine/17.0.308/src/web/src/engine/osk/src/visualKeyboard.ts:1295:40)
  at <anonymous> (https://s.keyman.com/kmw/engine/17.0.308/src/web/src/engine/osk/src/visualKeyboard.ts:1387:12)

[jahorton]

Sentry noted this after the merge of #11177 and #11140, as the code path returning null began being utilized by the changes in #11177.