modemmanager: handle no *.conf files being installed

To begin with, there are only a couple of .conf files, and
one of them is for testing, and the other is only installed
when MBIM is enabled, so if you build without MBIM you'll
have a failing install:

  install -m0644 /home/pprindeville/work/openwrt/build_dir/target-x86_64_musl/modemmanager-1.20.6/ipkg-install/usr/share/ModemManager/*.conf /home/pprindeville/work/openwrt/build_dir/target-x86_64_musl/modemmanager-1.20.6/.pkgdir/modemmanager/usr/share/ModemManager
  install: cannot stat '/home/pprindeville/work/openwrt/build_dir/target-x86_64_musl/modemmanager-1.20.6/ipkg-install/usr/share/ModemManager/*.conf': No such file or directory
  make[2]: *** [Makefile:161: /home/pprindeville/work/openwrt/build_dir/target-x86_64_musl/modemmanager-1.20.6/.pkgdir/modemmanager.installed] Error 1
  make[2]: Leaving directory '/home/pprindeville/work/openwrt/feeds/packages/net/modemmanager'

So make sure there's anything there to copy over first.

Signed-off-by: Philip Prindeville <pprindeville@netgate.com>