config: make empty js= omit script tag

According to the cgitrc man page, an empty js= value should cause the
script tag to be omitted. But instead, a script tag with an empty URL
is emitted. The same applies to css. So, skip emitting a tag if the
specified string is empty.

Signed-off-by: Samuel Lidén Borell <samuel@kodafritt.se>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>