Regression fix: Don't try to import empty-string custom target include dirs (#4342)
Regression fix: Don't try to import empty-string custom target include dirs (#4342)

* Don't try to import empty-string custom target include dirs

* Import current directory if custom target dir is empty

This restores the previous behavior and fixes test failures caused by
the previous commit.

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