mqv:filter is silently ignored due to operator precedence bug

[jona42-ui]

the mqv:filter is being "silently ignored" in cases where requestUri.pathname.slice(1) returns an empty string "" (which is falsy), but the nullish coalescing operator ?? only triggers on null or undefined, not on empty strings.

foexample:

URL: mqtt://broker.example.com/
Form has: { "mqv:filter": "sensors/+/temperature" }

Current behavior:

requestUri.pathname = "/"
requestUri.pathname.slice(1) = "" (empty string, not null/undefined)
filter = "" (mqv:filter is ignored!)

Expected behavior:

filter should be "sensors/+/temperature"