sqlite mandoc empty/failing apropos fix

[CauSeygn]

Added mandoc to the apropos . invocation rather than the failing apropos '' one.

This was tested on termux's sqlite3-backed mandoc, before this change qman would fail with:
'apropos .' failed; did you run mandb/makewhatis?
as if it was invoking that, meanwhile the real error was:

$ apropos ''
apropos: regcomp //: empty (sub)expression

Because in reality it was trying to do the other one which is fine on a flat-file mandoc but fails on the sqlite one; passing qman somemanpage in termux still succeeded without this fix despite the above, but there was no index nor navigable manpages within manpages inside of qman because apropos could not populate it, making it less than ideal to use.

The config file changes compared to stock config:

[misc]
system_type = mandoc
man_path = /data/data/com.termux/files/usr/bin/man
groff_path = /data/data/com.termux/files/usr/bin/groff
whatis_path = /data/data/com.termux/files/usr/bin/whatis
apropos_path = /data/data/com.termux/files/usr/bin/apropos
browser_path = /data/data/com.termux/files/usr/bin/xdg-open
mailer_path = /data/data/com.termux/files/usr/bin/xdg-open
viewer_path = /data/data/com.termux/files/usr/bin/xdg-open

Build instructions for termux:

1. meson is obtained via pip
   pip install meson
2. clone the repo and cd into it
3. setup the build for termux ($PREFIX is /data/data/com.termux/files/usr), compile and install qman:

# setup
meson setup build/ -Dpkg_config_path=/data/data/com.termux/files/usr/lib/pkgconfig -Dconfigdir=/data/data/com.termux/files/usr/etc/xdg/qman -Ddocdir=$PREFIX/usr/share/man -D bzip2:libraries=$PREFIX/lib/libbz2.so -D bzip2:include_directories=$PREFIX/include --prefix=$PREFIX
# compile
cd build/ && meson compile
# install
meson install

4. run qman and observe it being broken without this or in a less usable state, and fully working with this change.