fix RCTImageLoaderTests

Summary:
we couldn't run the RNTesterUnitTests before because these tests didn't respect the nullability specification. after this diff, we can run the tests.

as for the tests.... well 2 are still failing, i'm taking a look at them.

Changelog:
[iOS][Fixed] - fixed RCTImageLoaderTests

Reviewed By: sammy-SC

Differential Revision: D32635689

fbshipit-source-id: d68ae6a3e7f1370d7d76d68c7a6d9812928e6c12

Author: philIip

packages/rn-tester/RNTesterUnitTests/RCTImageLoaderTests.m

@@ -70,7 +70,7 @@ - (void)testImageLoading
         XCTAssertEqual(progress, 1);
         XCTAssertEqual(total, 1);
       }
-      partialLoadBlock:nil
+      partialLoadBlock:^(UIImage *loadedImage) {}
       completionBlock:^(NSError *loadError, id loadedImage) {
         XCTAssertEqualObjects(loadedImage, image);
         XCTAssertNil(loadError);
@@ -129,7 +129,7 @@ - (void)testImageLoaderUsesImageURLLoaderWithHighestPriority
         XCTAssertEqual(progress, 1);
         XCTAssertEqual(total, 1);
       }
-      partialLoadBlock:nil
+      partialLoadBlock:^(UIImage *loadedImage) {}
       completionBlock:^(NSError *loadError, id loadedImage) {
         XCTAssertEqualObjects(loadedImage, image);
         XCTAssertNil(loadError);