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
Merge "transform samples only when transformers exist"