Add a test which appends nothing to a property
When appending, we cannot remove properties
When appending, always pass non-NULL values
Test exporting a library without a target
Error when exporting a target without a language

First, it prevents a NULL dereference and second it reiterates that
targets without languages are not supported by CMake.
Merge topic 'dev/property-append-with-empty-string' into next

360eb20 When appending, always pass non-NULL values
b565e42 When appending, we cannot remove properties
54e4873 Add a test which appends nothing to a property
Merge topic 'dev/export-target-without-language' into next

654a840 Error when exporting a target without a language
8c808ab Test exporting a library without a target