meson: Fix pkg-config generation with internal dependencies

Prevents Meson error:

ERROR:  requires argument not a string, library with pkgconfig-generated
file or pkgconfig-dependency object, got <InternalDependency null: True>

Requires Meson 0.49.
2 jobs for meson-fallback in 3 minutes and 45 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #68843
build autotools

00:00:57

passed #68844
build meson

00:02:47