Config: Distinguish between empty and null strings

Config: Distinguish between empty and null strings

The C git API and command line tools distinguish between a key having
the empty string as a value and no key being present in the config
file:

$ echo -e '[a]\nx =' > foo.config
$ git config -f foo.config a.x; echo $?

0
$ git config -f foo.config a.y; echo $?
1

Make JGit make the same distinction. This is in line with the current
Javadoc of getString, which claims to return "a String value from the
config, null if not found". It is more reasonable to interpret "x ="
in the above example as "found" rather than "missing".

We need to maintain the special handling of a key name with no "="
resolving to a boolean true, but "=" with an empty string is still not
a valid boolean.

Change-Id: If0dbb7470c524259de0b167148db87f81be2d04a