[Go Feature Flag] null pointer if anonymous not set

[thomaspoignant]

We have a null pointer when the field anonymous is not set in the GO Feature Flag provider.

java-sdk-contrib/providers/go-feature-flag/src/main/java/dev/openfeature/contrib/providers/gofeatureflag/bean/GoFeatureFlagUser.java

Lines 35 to 36 in 11b5fe7

	Value anonymousValue = ctx.getValue(anonymousFieldName);
	boolean anonymous = anonymousValue.asBoolean();

This issue is here for me to remember to work on it.