shared/install: ignore enablement of template units w/o instance when presetting

When we have a unit which cannot be enabled:
 # foo@.service:
 ...
 [Install]
 WantedBy=foo.target  # there is no instance, so we don't know what to enable

we should throw an error when invoked directly with 'enable', but
not when doing 'preset' or 'preset-all'.

Fixes #19856.