Make prebuilt_apex report an error if no apex file is found

Previously, if an appropriate src property was not specified it would
return "" which resolves to the top level directory. This change causes
it to report an error.

Bug: 181267622
Test: m droid
Change-Id: Ia5be324a0eff18e43b352d71c6768c8767986053

apex/apex_test.go

@@ -4316,6 +4316,14 @@ func TestPrebuilt(t *testing.T) {
 	}
 }
 
+func TestPrebuiltMissingSrc(t *testing.T) {
+	testApexError(t, `module "myapex" variant "android_common".*: prebuilt_apex does not support "arm64_armv8-a"`, `
+		prebuilt_apex {
+			name: "myapex",
+		}
+	`)
+}
+
 func TestPrebuiltFilenameOverride(t *testing.T) {
 	ctx := testApex(t, `
 		prebuilt_apex {