return 'mixed' if the user mixes nucleotide and protein sequences
return 'mixed' if the user mixes nucleotide and protein sequences

SS would previously return `undefined` which was also return value for no or
small sequence.  Differentiating between the two would let SS appropriately
notify user.

Signed-off-by: Anurag Priyam <6e6ab5ea9cb59fe7c35d2a1fc74443577eb60635@gmail.com>

lepbase/lepbase-sequenceserver,IsmailM/sequenceserver,yeban/sequenceserver,elsiklab/sequenceserver,yeban/sequenceserver,vivekiitkgp/sequenceserver,aldermossy/sequenceserver,yeban/sequenceserver,elsiklab/sequenceserver,IsmailM/sequenceserver,aldermossy/sequenceserver,deepakunni3/sequenceserver,elsiklab/sequenceserver,aldermossy/sequenceserver,vivekiitkgp/sequenceserver,yeban/sequenceserver,lepbase/lepbase-sequenceserver,deepakunni3/sequenceserver,deepakunni3/sequenceserver,aldermossy/sequenceserver,jasminZ/sequenceserver,IsmailM/sequenceserver,jasminZ/sequenceserver,jasminZ/sequenceserver,vivekiitkgp/sequenceserver,lepbase/lepbase-sequenceserver