transform samples only when transformers exist

the pipeline is oddly setup to loop through each sample and
attempt to transform it even when there are no transformers. this
is madness and should be stopped -- this patch does this.

at best, this should improve performance because of needless
looping, at worst, we get rid of misleading 'tranforming sample'
logs.

Change-Id: I80ba2b7775fda03670c7fa556ad5a779c6eefd27
Validate AdvEnum & return an InvalidInput on error

Raise a relevant exception of InvalidInput while validating if
the input to AdvEnum is not one of what is expected.

Closes-bug: #1320430
Change-Id: I48f014cc09e7d88e9b0f02fb8e9cf45772178b2c
Merge "transform samples only when transformers exist"
Merge "Validate AdvEnum & return an InvalidInput on error"