don't export systemserverclasspath_fragment if contents are empty

If a systemserverclasspath_fragment only contains libraries that have a
higher min_sdk_version than the target build release version, then we
should not export the systemserverclasspath_fragment. Before this
change, the fragment was exported with an empty `contents` property
which caused errors after being dropped as a prebuilt.

Bug: 289183551
Test: go test ./sdk
Change-Id: Ifefc6880228e4dd37f5e42b2bda31a83df785375

sdk/update.go

@@ -455,11 +455,14 @@ be unnecessary as every module in the sdk already has its own licenses property.
 
 	for _, module := range builder.prebuiltOrder {
 		// Prune any empty property sets.
-		module = module.transform(pruneEmptySetTransformer{})
+		module = transformModule(module, pruneEmptySetTransformer{})
 
 		// Transform the module module to make it suitable for use in the snapshot.
-		module.transform(snapshotTransformer)
-		bpFile.AddModule(module)
+		module = transformModule(module, snapshotTransformer)
+		module = transformModule(module, emptyClasspathContentsTransformation{})
+		if module != nil {
+			bpFile.AddModule(module)
+		}
 	}
 
 	// generate Android.bp
@@ -835,9 +838,11 @@ type snapshotTransformation struct {
 }
 
 func (t snapshotTransformation) transformModule(module *bpModule) *bpModule {
-	// If the module is an internal member then use a unique name for it.
-	name := module.Name()
-	module.setProperty("name", t.builder.snapshotSdkMemberName(name, true))
+	if module != nil {
+		// If the module is an internal member then use a unique name for it.
+		name := module.Name()
+		module.setProperty("name", t.builder.snapshotSdkMemberName(name, true))
+	}
 	return module
 }
 
@@ -850,6 +855,25 @@ func (t snapshotTransformation) transformProperty(_ string, value interface{}, t
 	}
 }
 
+type emptyClasspathContentsTransformation struct {
+	identityTransformation
+}
+
+func (t emptyClasspathContentsTransformation) transformModule(module *bpModule) *bpModule {
+	classpathModuleTypes := []string{
+		"prebuilt_bootclasspath_fragment",
+		"prebuilt_systemserverclasspath_fragment",
+	}
+	if module != nil && android.InList(module.moduleType, classpathModuleTypes) {
+		if contents, ok := module.bpPropertySet.properties["contents"].([]string); ok {
+			if len(contents) == 0 {
+				return nil
+			}
+		}
+	}
+	return module
+}
+
 type pruneEmptySetTransformer struct {
 	identityTransformation
 }