Fix BlobModule getConstants()

Summary:
It's possible for us to return no constants from the BlobModule. Therefore, I'm correcting the flow-type.

Changelog:
[Internal]

Reviewed By: fkgozali

Differential Revision: D18932328

fbshipit-source-id: 2b415d12effd16eda313d5591825c711a20f9ae3

Author: RSNara

Libraries/Blob/NativeBlobModule.js

@@ -14,7 +14,7 @@ import type {TurboModule} from '../TurboModule/RCTExport';
 import * as TurboModuleRegistry from '../TurboModule/TurboModuleRegistry';
 
 export interface Spec extends TurboModule {
-  +getConstants: () => {|BLOB_URI_SCHEME: string, BLOB_URI_HOST: ?string|};
+  +getConstants: () => {|BLOB_URI_SCHEME: ?string, BLOB_URI_HOST: ?string|};
   +addNetworkingHandler: () => void;
   +addWebSocketHandler: (id: number) => void;
   +removeWebSocketHandler: (id: number) => void;

ReactAndroid/src/main/java/com/facebook/fbreact/specs/NativeBlobModuleSpec.java

@@ -55,11 +55,10 @@ public NativeBlobModuleSpec(ReactApplicationContext reactContext) {
   public final @Nullable Map<String, Object> getConstants() {
     Map<String, Object> constants = getTypedExportedConstants();
     if (ReactBuildConfig.DEBUG || ReactBuildConfig.IS_INTERNAL_BUILD) {
-      Set<String> obligatoryFlowConstants = new HashSet<>(Arrays.asList(
-          "BLOB_URI_SCHEME"
-      ));
+      Set<String> obligatoryFlowConstants = new HashSet<>();
       Set<String> optionalFlowConstants = new HashSet<>(Arrays.asList(
-          "BLOB_URI_HOST"
+          "BLOB_URI_HOST",
+          "BLOB_URI_SCHEME"
       ));
       Set<String> undeclaredConstants = new HashSet<>(constants.keySet());
       undeclaredConstants.removeAll(obligatoryFlowConstants);

ReactAndroid/src/main/java/com/facebook/react/modules/blob/BlobModule.java

@@ -163,7 +163,7 @@ public String getName() {
     String packageName = getReactApplicationContext().getPackageName();
     int resourceId = resources.getIdentifier("blob_provider_authority", "string", packageName);
     if (resourceId == 0) {
-      return null;
+      return MapBuilder.<String, Object>of();
     }
 
     return MapBuilder.<String, Object>of(