RFR: 8283801: Cleanup confusing String.toString calls

[Andrey Turbanov]

String.toString() calls doesn't make much sense. Only one place, where it could be used - to generate NPE. But in a few places of JDK codebase it's called, even when NPE will happen anyway.
I propose to cleanup such places.
Found by IntelliJ IDEA inspection `Redundant 'String' operation`.

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Commit messages:
 - [PATCH] Cleanup confusing String.toString calls

Changes: https://git.openjdk.java.net/jdk/pull/7878/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7878&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283801
  Stats: 14 lines in 4 files changed: 0 ins; 4 del; 10 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7878.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7878/head:pull/7878

PR: https://git.openjdk.java.net/jdk/pull/7878