coredata: fix bugs reading PKG_CONFIG_PATH
coredata: fix bugs reading PKG_CONFIG_PATH

There are two distinct bugs here:
1) if PKG_CONFIG_PATH is unset then the variable will be set to [''],
   which causes it to have the wrong behavior in some truthiness tests
2) We read PKG_CONFIG_PATH on invocations after the first, which causes
   it to overwrite the value saved into the coredata.dat file. Putting
   the two bugs together it means that -Dpkg_config_path doesn't work
   with `meson configure`

becm/meson,mesonbuild/meson,mesonbuild/meson,jpakkane/meson,becm/meson,QuLogic/meson,jpakkane/meson,jpakkane/meson,becm/meson,QuLogic/meson,jpakkane/meson,pexip/meson,jpakkane/meson,mesonbuild/meson,pexip/meson,QuLogic/meson,QuLogic/meson,QuLogic/meson,mesonbuild/meson,becm/meson,becm/meson,QuLogic/meson,becm/meson,mesonbuild/meson,pexip/meson,pexip/meson,QuLogic/meson,pexip/meson,jpakkane/meson,QuLogic/meson,becm/meson,jpakkane/meson,mesonbuild/meson,becm/meson,pexip/meson,pexip/meson,pexip/meson,jpakkane/meson,jpakkane/meson,QuLogic/meson,becm/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,becm/meson,pexip/meson,pexip/meson,mesonbuild/meson