RFR: 8371960: Missing null check in AnnotatedType annotation accessor methods

[Chen Liang]

`AnnotatedElement` methods specify they throw NPE if they take any null argument: `isAnnotationPresent`, `get(Declared)Annotation`, `get(Declared)AnnnotationsByType`. However, `AnnotatedType` has implementation quality issues that it does not throw NPE for `isAnnotationPresent` and `get(Declared)Annotation`.

This patch fixes this oversight by adding explicit null checks. For `get(Declared)AnnotationsByType`, even though it did throw exceptions, it was in the middle of parsing, so throwing it in the beginning is better.

-------------

Commit messages:
 - Null checks for AnnotatedType

Changes: https://git.openjdk.org/jdk/pull/28341/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28341&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8371960
  Stats: 105 lines in 2 files changed: 103 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/28341.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28341/head:pull/28341

PR: https://git.openjdk.org/jdk/pull/28341