Fix#7609, "error: unable to create file tests/ref/hello_a?foo#bar.html
(Invalid argument)" during git checkout on Windows.
Behavior change: passing an nonexistent file name on the command line
now shows a blank page (like network errors)
rather than exit with an error message.
I was concerned for a little bit that passing just whitespace to the
split_html_space_chars function would result in len > 0, so I wrote this
small unit test to make sure it does the right thing. Even though it
does do the right thing, I think there's still value in committing the
unit test.
