Fixes streams field.choice form dropdown value - Issue #2729

This was depending on entry_id but it was null =(
removed redundant isset
Treat 0 or '0' as nonempty values
Merge pull request #2730 from ryun/patch-5

Fixes streams field.choice form dropdown value - Issue #2729
Allow get_straem_fields to prefix the field slugs.

This is needed for grid to bypass something really hacky it was doing
when creating the input grid.