fix(fetch-http-handler): handle empty blob in browsers

[AllanZhengYP]

Fixes #1198

This PR fixes the issue with returning none base64 encoded data for empty blob in browser.

The original readToBase64() in streamCollector from fetch-http-handler was migrated from stream-collector-native package which was only intended to be used in ReactNative environment. It handles empty blobs fine in ReactNative, but misses unit test for the case. When the stream-collector-native is merged into fetch-http-handler, it can also be used in browser runtime, which can have different behavior from different browser vendors. This results untested behavior in browser runtime.

/cc @Amplifiyer

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[AllanZhengYP]

Tested with with Amplify Storage component(code snippet below) locally on Firefox and Chrome:

  const { Storage, Amplify } = require("aws-amplify");
  Amplify.configure({
    Storage: {
      AWSS3: {
        bucket: "bucket",
        region,
        identityPoolId: IDENTITY_POOL_ID,
      },
    },
  });
  await Storage.put("abcd");

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