man.fish can be clarified a bit, by removing a superfluous early return. Additionally, performance can be
(ever so slightly) improved, by using the empty string to suffix an extra colon when `$MANPATH` is empty, as
described in `manpath(1)`. As `man` will internally call `manpath` as it starts, this eliminates a redundancy.
I hate doing this but I am tired of touching a fish script as part of
some change and having `make style` radically change it. Which makes
editing fish scripts more painful than it needs to be. It is time to do
a wholesale reformatting of these scripts to conform to the documented
style as implemented by the `fish_indent` program.
