gain_groups: Add zero-value gain groups

Add convenience factory for making a gain group that has a single
zero-valued element. This factory requires a name, which should
probably be ALL_GAINS, or something similar (these constants are
device-specific).

Using this new make_zero factory in the X300 radio control when we
don't find any gain elements so that our gain groups aren't empty. This
simplifies our later setters/getters because we know that we'll always
have _something_ cached.
Note that we only register this zero value gain group for TX, as our
ADC is registered as a gain element, so our RX gain groups are never
empty.

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