Handle shift-delete of empty line at beginning of prompt

[waf]

Closes #254

[codecov]

Codecov Report

Merging #263 (ba22a69) into main (45411cc) will increase coverage by 0.0%.
The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main    #263   +/-   ##
=====================================
  Coverage   85.1%   85.2%           
=====================================
  Files         45      45           
  Lines       3875    3880    +5     
  Branches     598     600    +2     
=====================================
+ Hits        3301    3306    +5     
  Misses       435     435           
  Partials     139     139           

Impacted Files	Coverage Δ
src/PrettyPrompt/Documents/Document.cs	93.0% <100.0%> (+<0.1%)	⬆️

... and 1 file with indirect coverage changes