Fix droiddoc disting when WITHOUT_CHECK_API is true.

When WITHOUT_CHECK_API=true, the droidstubs apiFile can be nil. Fix this
by defensively checking for nil paths when making the TaggedDistFiles,
and also in android.MakeDefaultDistFiles.

The error message reported in r.android.com/1335521 was:

"internal error: panic in GenerateBuildActions for singleton androidmk
  Dist file should not be nil for the default tag in
  android.net.ipsec.ike.stubs.source in translateAndroidMkModule for
  module android.net.ipsec.ike.stubs.source variant android_common"

Test: WITHOUT_CHECK_API=true m droid dist

Bug: 152834186
Signed-off-by: Jingwen Chen <jingwen@google.com>
Change-Id: I1b1f7c0b7a0e1c0ed5e15957d0162c47fd3ec197