Image component crashes on iOS when uri is null

[mlazari]

Description:

Rendering an Image with source={{ uri: null }} crashes on iOS with a JSON value '<null>' of type NSNull cannot be converted to NSString error. It seems to have started doing so in version 0.61.0 after this commit: df17ebf
Seems to be a regression of f940e7c

React Native version:

info Fetching system and libraries information...
System:
    OS: macOS 10.15
    CPU: (4) x64 Intel(R) Core(TM) i5-4258U CPU @ 2.40GHz
    Memory: 285.80 MB / 8.00 GB
    Shell: 5.7.1 - /bin/zsh
  Binaries:
    Node: 10.15.1 - ~/.nvm/versions/node/v10.15.1/bin/node
    Yarn: 1.17.3 - /usr/local/bin/yarn
    npm: 6.13.0 - ~/.nvm/versions/node/v10.15.1/bin/npm
  SDKs:
    iOS SDK:
      Platforms: iOS 13.0, DriverKit 19.0, macOS 10.15, tvOS 13.0, watchOS 6.0
    Android SDK:
      API Levels: 26, 27, 28, 29
      Build Tools: 27.0.3, 28.0.2, 28.0.3
      System Images: android-27 | Google APIs Intel x86 Atom
  IDEs:
    Android Studio: 3.5 AI-191.8026.42.35.6010548
    Xcode: 11.0/11A420a - /usr/bin/xcodebuild
  npmPackages:
    react: ^16.10.2 => 16.12.0 
    react-native: ^0.61.5 => 0.61.5

Steps To Reproduce

1. Create a simple app that renders <Image source={{ uri: null }} />
2. Run the app on iOS
3. The app crashes with a JSON value '<null>' of type NSNull cannot be converted to NSString error.

Expected Results

The app should not crash.

Snack, code example, screenshot, or link to a repository:

<Image source={{ uri: null }} />