Warn instead of crashing on preexisting wrong data type
Warn instead of crashing on preexisting wrong data type

Using None in a VTYPE_MAYBE_STRING is wrong, because it cannot really
be serialized to a Haskel Nothing, but given that this can (and does)
influence external components already using None, it is turned to a
warning in order not to break the existing behavior.

Signed-off-by: Michele Tartara <05f7077a2c6cbbdc4af4ba42e3ef89a0df846179@google.com>
Reviewed-by: Helga Velroyen <2d0126b1ec6f9ec5cbec4a7012aa14faeaef10f5@google.com>

leshchevds/ganeti,apyrgio/ganeti,mbakke/ganeti,onponomarev/ganeti,grnet/snf-ganeti,ganeti/ganeti,apyrgio/ganeti,dimara/ganeti,yiannist/ganeti,apyrgio/snf-ganeti,ganeti/ganeti,andir/ganeti,yiannist/ganeti,mbakke/ganeti,bitemyapp/ganeti,andir/ganeti,mbakke/ganeti,grnet/snf-ganeti,ganeti-github-testing/ganeti-test-1,dimara/ganeti,leshchevds/ganeti,ganeti-github-testing/ganeti-test-1,bitemyapp/ganeti,andir/ganeti,onponomarev/ganeti,ganeti/ganeti,yiannist/ganeti,leshchevds/ganeti,apyrgio/snf-ganeti