RFR: 8283041: [javadoc] Crashes using {@return} with @param

[Pavel Rappo]

The inline `{@return}` tag is relatively new and will require developers to change their habits. According to the [specification](https://docs.oracle.com/en/java/javase/17/docs/specs/javadoc/doc-comment-spec.html#return), the inline version of `@return` "may only occur at the beginning of a method's description".

When used like in the description of the issue, the tag technically belongs to the block `@param` tag and not to the body of the doc comment, which one might think is the case. Thus, the "full body" (let alone "first sentence") collection of doc nodes is empty. Hence, IndexOutOfBoundsException when trying to access its first element. 

Since we don't have a method that returns the **complete** doc comment (yes, "getFullBody" is a bit of a misleading name), whose first element we could check against `{@return}`, I check `isEmpty()` before accessing the first element.

Interestingly, `{@summary}` (must also appear first) lint is performed differently. However, I decided not to copy it since it operates on a lower level of abstraction: characters and strings thereof.

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.java.net/jdk/pull/7788/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7788&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283041
  Stats: 19 lines in 2 files changed: 14 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7788.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7788/head:pull/7788

PR: https://git.openjdk.java.net/jdk/pull/7788