RFR: 8335610: DiagnosticFramework: CmdLine::is_executable() correction

[Kevin Walls]

CmdLine::is_executable() looks wrong, surely an empty line is not executable.

With this change, in DCmd::parse_and_execute() we will avoid needlessly entering the code block to try and execute the command.

DCmd tests all good:
make images test TEST="test/hotspot/jtreg/serviceability/dcmd test/jdk/sun/tools/jcmd"

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Commit messages:
 - DiagnosticFramework: CmdLine::is_executable() correction

Changes: https://git.openjdk.org/jdk/pull/20006/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20006&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8335610
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/20006.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20006/head:pull/20006

PR: https://git.openjdk.org/jdk/pull/20006