add blank line check

[incebellipipo]

In PR #24685 I made mistake and add a blank line at the bottom of the yaml file. @clalancette replied with #24685 (comment) and pointed out that blank line check is missing in linter script. So I add this functionality to check_rosdep.py script and update contributing guide accordingly.

Proposal: #24685 (comment)

Adding this will do.

def no_empty_lines(buf):
   clean = True 
   for i, l in enumerate(buf.split('\n')[:-1]):
       if re.match(r'^\s*$', l):
           print_err("empty line %u" % (i+1))
           clean = False
   return clean

Results:

checking for trailing spaces...
checking for empty lines...
 ERR: empty line 6225
checking for incorrect indentation...
checking for non-bracket package lists...
checking for item order...
building yaml dict...
there were errors, please correct the file

I also checked for a case where an empty line also has trailing whitespaces which is kinda rare. If that happens script returns this:

$ ./check_rosdep.py ../rosdep/base.yaml
checking for trailing spaces...
 ERR: trailing space line 6225
checking for empty lines...
 ERR: empty line 6225
checking for incorrect indentation...
 ERR: line 6224:   
Traceback (most recent call last):
 File "./check_rosdep.py", line 223, in <module>
   if not main(args.infile):
 File "./check_rosdep.py", line 181, in main
   my_assert(correct_indent(buf))
 File "./check_rosdep.py", line 116, in correct_indent
   return generic_parser(buf, fun)
 File "./check_rosdep.py", line 82, in generic_parser
   s = re.search(r'(?!' + indent_atom + ')[^\s]', l).start()
AttributeError: 'NoneType' object has no attribute 'start'
$ echo $?
1

I didn't want to commit my local change, since it will automatically be a part of this PR which labelled as rosdep. If its ok, I can commit it, if its not I'll wait for this PR to be merged and create a new PR.

[clalancette]

Two small things to fix, but otherwise looks good to me. Thanks for the change.