Timesyncd empty FallbackNTP does not clear the list

[Itxaka]

According to docs:

When the empty string is assigned, the list of NTP servers is reset, and all prior assignments will have no effect. If this option is not given, a compiled-in list of NTP servers is used.

so int he case of not setting it thats ok, but setting it to an empty string should make it clear the fallback list.

we got a report that this does not work and keeps the existing fallback server:

Also, the FallbackNTP: "" option does not work to clear the setting, it stays at [ntp.ubuntu.com](http://ntp.ubuntu.com/). The setting can however be used to set a different fallback server, just not an empty one.

[Itxaka]

Added a patch that improves it somehow by setting empty string for real for empty strings instead of omitting the value.

also moves the config to an override