RFR: JDK-8267219: Javadoc method summary breaks when {@inheritDoc} from an empty parent [v4]

[liach]

> This change fixes when a method body has only inline tags that produce no output, the method summary will get eaten.
> 
> This change allows `{@inheritDoc}` from empty parents to go through the code path used by `-nocomment` and properly generate tables.
> 
> All `jtreg:test/langtools/jdk/javadoc/doclet` tests pass.

liach has updated the pull request incrementally with one additional commit since the last revision:

  Specify further what isValid expects and how it is used

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4066/files
  - new: https://git.openjdk.java.net/jdk/pull/4066/files/6e4435cd..93c50905

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4066&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4066&range=02-03

  Stats: 10 lines in 2 files changed: 9 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4066.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4066/head:pull/4066

PR: https://git.openjdk.java.net/jdk/pull/4066