Work around `art-host-tests` build issues when using ART prebuilts.

Using prebuilts of the ART Module seems to make
`COMPATIBILITY.art-host-tests.HOST_SHARED_LIBRARY.FILES` empty on
x86 targets, thus breaking the `art-host-tests` build target. As
a workaround, relax the corresponding build rule to allow for an
empty `COMPATIBILITY.art-host-tests.HOST_SHARED_LIBRARY.FILES`
list.

(cherry picked from commit 4d9d0aa8)

Test: lunch cf_x86_phone-userdebug
        && SOONG_CONFIG_art_module_source_build=false m art-host-tests
Bug: 194627489
Change-Id: I9e885be3c7161f6f09a93b3d32339a5a6e57d2a1
Merged-In: I9e885be3c7161f6f09a93b3d32339a5a6e57d2a1