Don't panic in turbine with no classes

ALLOW_MISSING_DEPENDENCIES=true can cause missing dependencies to be
hidden from GenerateAndroidBuildActions, which is triggering the
len(jars) == 0 check in compileJavaHeader.  For the
ALLOW_MISSING_DEPENDENCIES=true case the check is unnecessary, as the
build rules will be replaced with runtime errors.  On the off chance
the check would have been hit in a real build, allow merge_zips to
create an empty zip file if it is asked to.

Test: m ALLOW_MISSING_DEPENDENCIES=true
Change-Id: I3171f921d51229ddf38a1a647d32566658c673fa

cmd/merge_zips/merge_zips.go

@@ -78,7 +78,7 @@ func main() {
 	// parse args
 	flag.Parse()
 	args := flag.Args()
-	if len(args) < 2 {
+	if len(args) < 1 {
 		flag.Usage()
 		os.Exit(1)
 	}

java/java.go

@@ -689,15 +689,11 @@ func (j *Module) compileJavaHeader(ctx android.ModuleContext, srcFiles android.P
 	var headerJar android.Path
 	jars = append(jars, deps.staticHeaderJars...)
 
-	if len(jars) == 0 {
-		panic("The turbine.jar is empty without any sources and static libs.")
-	} else {
 	// we cannot skip the combine step for now if there is only one jar
 	// since we have to strip META-INF/TRANSITIVE dir from turbine.jar
 	combinedJar := android.PathForModuleOut(ctx, "turbine-combined", jarName)
 	TransformJarsToJar(ctx, combinedJar, "for turbine", jars, android.OptionalPath{}, false, []string{"META-INF"})
 	headerJar = combinedJar
-	}
 
 	if j.properties.Jarjar_rules != nil {
 		jarjar_rules := android.PathForModuleSrc(ctx, *j.properties.Jarjar_rules)