Fix missing source module with ALLOW_MISSING_DEPENDENCIES=true

A module referenced with srcs: ":module" was not honoring
ALLOW_MISSING_DEPENDENCIES=true.  Don't fail if the module
doesn't exist, an error will already have been produced by
ExtractSourcesDeps.

Bug: 68183622
Test: m ALLOW_MISSING_DEPENDENCIES=true
Change-Id: Id806633d12ab2ecd78b532e1922e59824e5c20a8

android/module.go

@@ -982,6 +982,10 @@ func (ctx *androidModuleContext) ExpandSourcesSubDir(srcFiles, excludes []string
 	for _, s := range srcFiles {
 		if m := SrcIsModule(s); m != "" {
 			module := ctx.GetDirectDepWithTag(m, SourceDepTag)
+			if module == nil {
+				// Error will have been handled by ExtractSourcesDeps
+				continue
+			}
 			if srcProducer, ok := module.(SourceFileProducer); ok {
 				expandedSrcFiles = append(expandedSrcFiles, srcProducer.Srcs()...)
 			} else {