[web] getFontSizeStyle returns null for valid font sizes

[mcdurdin]

Describe the bug
Function getFontSizeStyle returns null when passed valid font sizes such as "12pt". Callers of this function don't test the result so this results in an error when, for example, a .keyman-touch-layout file specifies a "fontsize": "12" or "fontsize": "12pt".

Uncaught TypeError: Cannot read property 'absolute' of null
    at OSKBaseKey.OSKKey.generateKeyText (kmwosk.ts:197)
    at OSKBaseKey.construct (kmwosk.ts:338)
    at Object.osk.deviceDependentLayout (kmwosk.ts:2748)
    at Object.osk._GenerateVisualKeyboard (kmwosk.ts:3642)
    at Object.osk._Load (kmwosk.ts:4741)
    at HTMLScriptElement.<anonymous> (kmwkeyboards.ts:648)

Screenshots

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KeymanWeb:

• KeymanWeb Version: 11.0