Allow target commands to be invoked with no items (#14325).

Code such as

 target_include_directories(foo PRIVATE ${items})

should not work or break based on whether items is defined or not.
Merge topic 'target-command-allow-no-items' into next

c0b8682 Allow target commands to be invoked with no items (#14325).
Revert "Docs: Document that target property commands require at least one item."

This reverts commit 452e58525bb1656df0a169c8a047c25837caa4cf.
Merge topic 'fix-target-property-command-docs' into next

e79ab6a Revert "Docs: Document that target property commands require at least one item."