trailing whitespace in indented blocks

[casperdcl]

blank lines within an indented block should be blank rather than indented (pure whitespace).

diff (-before +after):

 if 1
     display('first line, with a blank line below');
-    
+
     display('second line, with a blank line above');
 end

this will also make MBeautify compatible with https://github.com/pre-commit/pre-commit-hooks#trailing-whitespace and many code editors which auto-strip trailing whitespace regardless of indentation context.

Looks like this is the line which needs to be implemented:

https://github.com/davidvarga/MBeautifier/blob/4087074c1221c3748c29278fb0cca1c6f3681605/%2BMBeautifier/MIndenter.m#L47-L48