RFR 8224114: Implement Scope.noFilter instead of using a null reference

[Ron Shapiro]

Please review this small refactoring to implement Scope.noFilter instead of
maintaining several null checks throughout the code.

bug: https://bugs.openjdk.java.net/browse/JDK-8224114
webrev: http://cr.openjdk.java.net/~ronsh/8224114/webrev.00/
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