Ensure that taxonomy name passed to `get_term()` filters is non-empty.

Since [34997], the `$taxonomy` parameter of `get_term()` has been
optional. This created cases where the `$taxonomy` parameter, used
to concatenate names for some filters and passed as a parameter to
others, would be empty. This changeset ensures that it's never
empty by falling back on the `taxonomy` of the located term.

Props dlh.
Fixes #45698.
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