export: Restore support for empty TARGETS list

Refactoring in commit f5acecaa (cmExportCommand: Port to
cmArgumentParser, 2019-03-23, v3.15.0-rc1~270^2~3) broke the `export`
command's support for specifying `TARGETS` with no entries.  Fix it and
add a test case.

Fixes: #19415
Status Job ID Name Coverage
  External
passed cdash

passed cmake-hythloth-linux-debug-makefiles+sphinx

00:44:42

passed cmake-minoc-linux-release-lint-iwyu-tidy

00:12:08

passed cmake-moonshade-osx-debug-ninja

00:10:00

passed cmake-moonshade-osx-release-makefiles

00:11:08

passed cmake-terfin-windows-release-vs2019

00:44:53

passed ghostflow-check-master

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager