[java] Suggest a new rule for uninitialized non final member class/ class variable

[jmetertea]

Affects PMD Version:
Add new rule to next version
Rule:
I'm suggesting adding a new rule UnusedMemberClassVariableRule
Description:
Java compiler isn't warning on a non final private member class/ class variable that has no initialization code and is used (thus create a NullPointerException)
Relevant question: https://stackoverflow.com/questions/52810496/avoid-uninitialized-non-final-member-class-class-variable

**Code Sample demonstrating the issue:**
private static Map<String, String> mapStatic; // Missing compile/warning
private Map<String, String> map; // Missing compile/warning
public void  put() {
    map.put("x", "1"); // NullPointerException
    //Map<String, String> mapVar;
    //mapVar.put("x", "1"); // Compile The local variable mapVar may not have been initialized
}
static {
    mapStatic.put("x", "1"); // NullPointerException        
}

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