RFR: 7121: Unhandled exception while working with Thread Graphs

[Alex Macdonald]

This short PR addresses JMC-7121 [0], in which a  division by 0 error can occur when viewing the threads page with a focused selection that has no relevant thread information. A big thanks to Bipin for catching this, but for also providing easy steps to reproduce the issue.

The fix here adds a check when initializing the minimum lane height, ensuring that the default value gets set if there are no items.

[0] https://bugs.openjdk.java.net/browse/JMC-7121

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Commit messages:
 - 7121: Unhandled exception while working with Thread Graphs

Changes: https://git.openjdk.java.net/jmc/pull/210/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=210&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7121
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jmc/pull/210.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/210/head:pull/210

PR: https://git.openjdk.java.net/jmc/pull/210