RFR: 8325936: jshell - crash on 'new Object().""'

[Hannes Greule]

This fixes a crash in jshell when the target of a StringTemplate is not a processor. We only look up the `process` method if the type is actually a processor. The added test case fails without that fix.

Please let me know what you think of this fix, and if there are things that should be changed.

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Commit messages:
 - cleanup
 - add test case
 - Simplify, add test
 - Don't expect confusing errors
 - Fix fatal error when string template target is not a processor

Changes: https://git.openjdk.org/jdk/pull/17876/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17876&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325936
  Stats: 29 lines in 4 files changed: 12 ins; 10 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/17876.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17876/head:pull/17876

PR: https://git.openjdk.org/jdk/pull/17876