RFR: 8277536: Use String.blank in jdk.javadoc where applicable

[Andrey Turbanov]

There are couple of places where manual cycle is used to check if `String` contains non-whitespaces characters.
Since Java 11 we have String.isBlank() which could be used instead.

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Commit messages:
 - [PATCH] Use String.isBlank in jdk.javadoc
 - [PATCH] Remove redundant boxing in jdk.javadoc.internal.doclint.Checker

Changes: https://git.openjdk.java.net/jdk/pull/6323/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6323&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277536
  Stats: 20 lines in 2 files changed: 0 ins; 14 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6323.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6323/head:pull/6323

PR: https://git.openjdk.java.net/jdk/pull/6323