[improve][cli] Pulsar shell: fix custom commands execution

[nicoloboschi]

Motivation

In Pulsar shell the custom commands don't work (PIP-201)

java.lang.NullPointerException: Cannot invoke "java.lang.Class.getConstructor(java.lang.Class[])" because the return value of "java.util.Map$Entry.getValue()" is null
        at org.apache.pulsar.admin.cli.PulsarAdminTool.addCommand(PulsarAdminTool.java:218)
        at org.apache.pulsar.admin.cli.PulsarAdminTool.setupCommands(PulsarAdminTool.java:164)
        at org.apache.pulsar.admin.cli.PulsarAdminTool.setupAndRun(PulsarAdminTool.java:224)
        at org.apache.pulsar.admin.cli.PulsarAdminTool.run(PulsarAdminTool.java:229)
        at org.apache.pulsar.shell.AdminShell.runCommand(AdminShell.java:72)
        at org.apache.pulsar.shell.PulsarShell.run(PulsarShell.java:436)
        at org.apache.pulsar.shell.PulsarShell.run(PulsarShell.java:220)
        at org.apache.pulsar.shell.PulsarShell.main(PulsarShell.java:209)

We just need to avoid the NPE because sometimes the setupCommands is called twice for prepare Pulsar shell commands for the autocompletion

Modifications

• Avoid NPE

Verifying this change

Verify both autocompletion and execution work correctly

Documentation

• doc-not-needed