fix: Tagging is working again

[micz]

Tagging is currently giving this error:

Uncaught (in promise) Error: Type error for parameter newProperties (Error processing tags.0: Expected string instead of undefined) for messages.update.
    applyTags moz-extension://e7252b71-5fcf-458f-af21-905f604adfad/background.js:117
    applyTags moz-extension://e7252b71-5fcf-458f-af21-905f604adfad/background.js:102
    spinWith moz-extension://e7252b71-5fcf-458f-af21-905f604adfad/background.js:31
    <anonymous> moz-extension://e7252b71-5fcf-458f-af21-905f604adfad/background.js:181
    load moz-extension://e7252b71-5fcf-458f-af21-905f604adfad/popup/popup.js:186
    dispatchSelect moz-extension://e7252b71-5fcf-458f-af21-905f604adfad/popup/baseItemList.js:378
    itemListClick moz-extension://e7252b71-5fcf-458f-af21-905f604adfad/popup/baseItemList.js:368
    connectedCallback moz-extension://e7252b71-5fcf-458f-af21-905f604adfad/popup/baseItemList.js:247
    <anonymous> moz-extension://e7252b71-5fcf-458f-af21-905f604adfad/popup/tagList.js:27
background.js:117:31

this PR fixes the issue.

[kewisch]

lgtm, thanks!