NullPointerException if LottieValueCallback returns null.

[idunnololz]

Describe the bug
LottieValueCallback is allowed to return null to indicate the default value should be used as it's documentation states (

lottie-android/lottie/src/main/java/com/airbnb/lottie/value/LottieValueCallback.java

Line 51 in 664c8e3

public T getValue(LottieFrameInfo<T> frameInfo) {

).

The commit a808c3f introduced a crash.

The offending commit adds the following code to SolidLayer.java:

if (colorAnimation != null) {
  paint.setColor(colorAnimation.getValue());
}

colorAnimation can be backed by a LottieValueCallback. Thus colorAnimation.getValue() can return null. When this happens
paint.setColor(colorAnimation.getValue()); will crash with a NPE as it tries to dereference a null boxed type (Integer).

What version of Lottie did you test this on?
v6.2.0.

What version of Android did you test this on?
This crash occurs on all versions of Android that Lottie supports.

Steps To Reproduce
Steps to reproduce the behavior:

1. Add a LottieAnimationView to any screen.
2. Add a color value callback to the LottieAnimationView that just returns null for the value. Eg. (kotlin)

      myLottieAnimationView.addValueCallback(
        KeyPath("**"),
        LottieProperty.COLOR,
        object : LottieValueCallback<Int>() {
          override fun getValue(frameInfo: LottieFrameInfo<Int>?): Int? {
            return null
          }
        },
      )

4. Load any lottie file with a solid layer. I've attached an example lottie file here:
   lottie.json