Ignore empty ("_") CV on Icinga 2 export.

Refs #2400
Icinga 2 Export: Add backwards compatibility for lconfcheckcommand.

Refs #2392
Merge branch 'fix/checkcommand-2392' into support/1.4

Fixes #2392
Merge branch 'fix/empty-cv-key-2400' into support/1.4

Fixes #2400
Icinga 2 Export: Host event_commands are not supported.

Refs #2394
Merge branch 'fix/checkcommand-2392' into support/1.4

Fixes #2392
Merge branch 'fix/empty-cv-key-2400' into support/1.4

Fixes #2400
Merge branch 'fix/host-eventcmd-2394' into support/1.4

Fixes #2394