document handling of empty X resources values

since 5cab2fb we reject empty X resources value to support use-cases
like this: https://codeberg.org/nsxiv/nsxiv/issues/339
this issue was also bought up by GRFreire in:
https://codeberg.org/nsxiv/nsxiv-record/pulls/115#issuecomment-474831

this suggests that this is a use-case that a decent amount of users
might be interested in. so document the behavior so it's more easily
visible.

etc/nsxiv.1

@@ -457,6 +457,9 @@ Color of the bar foreground. Defaults to window.foreground
 Color of the mark foreground. Defaults to window.foreground
 .TP
 Please see xrdb(1) on how to change them.
+.LP
+An X resources entry with an empty value means the default
+(defined in config.h) will be used.
 .SH WINDOW TITLE
 The window title can be replaced with the output of a user-provided script,
 which is called by nsxiv whenever any of the relevant information changes.