utils: let make_ui return an empty ui.ui() object if the requested hgrc file isn't present

89c30b145bb8 lost an "if repoui" in simplehg, and Kallithea would thus fail on
serving repos without a .hg/hgrc :

File ".../kallithea/lib/middleware/simplehg.py", line 285, in _augment_hgrc
for k, v in repoui.configitems(section):
AttributeError: 'bool' object has no attribute 'configitems'

It is *only* simplehg that invoke make_ui with read_from 'file'. In this case
it could return False if the requested file wasn't present.

It was odd and inconsistent that make_ui with read_from 'file' either returned
an ui object or the value False.

To fix the problem, just let make_ui return the empty ui.ui() if no .hg/hgrc is
present. An ui object that just doesn't have any config sections is much better
than any alternative.