RFR: 8263248: IGV: accept graphs without node categories

Wed Mar 10 08:42:22 UTC 2021

Currently, IGV fails if the input graphs do not contain node category information (added in JDK 17). This change replaces the failing assertion with a warning message in the log file, to support using the latest IGV on graphs generated before JDK 17.

Tested manually on graphs generated [from JDK 11](https://bugs.openjdk.java.net/secure/attachment/93600/from-jdk-11.zip) and [from JDK 17](https://bugs.openjdk.java.net/secure/attachment/93601/from-jdk-17.zip).

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Commit messages:
 - 8263248: IGV: accept graphs without node categories

Changes: https://git.openjdk.java.net/jdk/pull/2909/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2909&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263248
  Stats: 27 lines in 2 files changed: 24 ins; 3 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2909.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2909/head:pull/2909

PR: https://git.openjdk.java.net/jdk/pull/2909