RFR: 7903151: NPE when resolving @library from jtreg plugin

[Maurizio Cimadamore]

This pathc fixes a trivial NPE which occurs when the plugin is attempting to resolve an absolute library in a `@library` tag. If no library is found starting from TEST.ROOT, and if TEST.ROOT does not specify any alternate library root, the plugin fails with a NPE, as the code is eagerly trimming the property value "external.lib.roots", which might not exist.

The solution is simple, e.g. check for null before trimming.

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Commit messages:
 - Initial push

Changes: https://git.openjdk.java.net/jtreg/pull/70/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jtreg&pr=70&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7903151
  Stats: 5 lines in 3 files changed: 2 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jtreg/pull/70.diff
  Fetch: git fetch https://git.openjdk.java.net/jtreg pull/70/head:pull/70

PR: https://git.openjdk.java.net/jtreg/pull/70