blockimgdiff.py: Fix the script error for BBOTA v1.

CL in [1] added an assertion to make sure an empty RangeSet cannot call
RangeSet.to_raw_string(). However, we may unintentionally hit that when
generating BBOTA v1 script (e.g. for "new" commands that always have
empty src_ranges).

[1] commit e8f75613

Change-Id: I1723606967f354ace9286d2b888fdbc80ba10e39