Avoid redundant HashMap.containsKey call in java.util.regex

Pattern.namedGroups and Matcher.namedGroups contains only non-null values. It means instead of separate containsKey+get calls, we can use single HashMap.get call and then compare result with null.
Result code is a bit simpler and faster.

Affected methods:
1. java.util.regex.Matcher#getMatchedGroupIndex
2. java.util.regex.Pattern#escape
3. java.util.regex.Matcher#appendExpandedReplacement