pkgconfig throws when trying to substitute empty variables

Hello,

if a pkg-config file contains an empty variable which will we used by another variable or key the Qbs pkgconfig-parser just throws an error. But it is legal to have and use empty variables in a pkgconfig file.

In my case my systemd.pc file contains

root_prefix=
...
systemd_util_dir=${root_prefix}/lib/systemd

The line triggering the exception: https://github.com/qbs/qbs/blob/b20256ccef06f515d1cbfdd51bf2b2398d114c70/src/lib/pkgconfig/pcparser.cpp#L469

As I understand it is completely valid to reference a undefined variables. Thus the result here beeing an empty string would be valid. No error should be thrown.

(see https://github.com/pkgconf/pkgconf/blob/b0802cb3d19b5b963ad20eed464a2da6e7d7b0b5/libpkgconf/tuple.c#L293 for reference)