Add empty line for whitespace-only comment markers

When a license header line is empty and comment_style.middle
contains only whitespace characters, add a truly empty line
instead of the whitespace-only marker. This improves the
formatting of license headers.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

plugin/licenser.vim

@@ -113,7 +113,11 @@ function! s:AddLicenseHeader(license) abort
 		let line = substitute(line, 'AUTHOR', author, 'g')
 
 		if line ==# ''
+			if comment_style.middle =~# '^\s*$'
+				call add(processed_lines, '')
+			else
 				call add(processed_lines, comment_style.middle)
+			endif
 		else
 			call add(processed_lines, comment_style.middle . ' ' . line)
 		endif