The code was meant to leave the first row empty for the text, but ended up drawing the "histogram" one row shorter

- hammer-parse-apply wasn't clearing the pointer to the desired stack frame
- Reproduction steps: hammer-parse-step N, hammer-parse-apply, hammer-parse-step 1

- Also fixes double curly braces when printing HByte structs when len is 0

Add tests for .set_top_node(0, None), and .set_top_node(None, None)

Also ensure cleanup runs in HParseResult test