Updated torbrowser_version number is not enough to change firefox-android versionCode number

When a new Tor Browser version doesn't include any new firefox-android commit, its versionCode number will not change, although MOZ_BUILD_DATE is changing.

The reason is that the function generateFennecVersionCode in fenix/buildSrc/src/main/java/Config.kt is taking the date from MOZ_BUILD_DATE and using the number of hours from December 28, 2014, to generate a versionCode. However, MOZ_BUILD_DATE is based on the commit date, on which we add the Tor Browser version as seconds, so when a new version doesn't change the firefox-android, MOZ_BUILD_DATE will only be change by a few seconds, which is not enough to make generateFennecVersionCode update the versionCode.

To fix this we can add an empty firefox-android commit as a quick workaround for this release, before finding a better way to handle changing MOZ_BUILD_DATE for new releases.