Blank space in a Nemeth table does present a dilemma to the transcriber.
Yes, inside the switches, you use Nemeth symbols when the missing or blank entry is printed as a dash, underscore, etc. But when it is blank space, we are now suggesting that you follow Braille Formats and fill out the width of the column with guide dots (dot 5's). It doesn't matter if the blank space indicates a missing answer, or if it represents "no entry". By simply representing the blank entry with guide dots, you will not need to decide whether or not that space is to be filled in with an answer.
A transcriber's note is required to tell the reader that "A series of guide dots across the width of the column indicates a blank space." You don't need to say "a blank space to be filled in". You are just playing it as it lays.
Note that guide dots can appear inside a Nemeth table without needing to switch codes. Just like page change indicators, box lines, and column separation lines, guide dots can occur in either code.