set-property should add the property if it does not yet exist

[famod]

set-property should add the respective property if it does not exist physically in the current pom.xml.

Context: I have a certain property with a default value in the parent pom for my project and I want to override the property in projects which are using that parent pom (which is located in a separate repo).

It might be a good idea to make this an opt in.