Allow empty versions in alias definitions in catalog

[edudar]

I can add a library with or without a version to a catalog.

build.gradle:
catalog {
    versionCatalog {
        library('lib1', 'my', 'library1').version('1.0.0')
        library('lib2', 'my', 'library2').withoutVersion()
    }
}

---

toml:
[libraries]
lib1 = {group = "my", name = "library1", version = "1.0.0" }
lib2 = {group = "my", name = "library2", version = "" }

This works fine, and I can use lib1 right away or would need a platform that will provide a version for lib2. I can also override the lib1 version from settings.gradle. Now, I want to add a library without a version (aka needs a platform) and keep an option to set that version manually/override.

build.gradle:
catalog {
    versionCatalog {
        version('lib2-version', '')

        library('lib1', 'my', 'library1').version('1.0.0')
        library('lib2', 'my', 'library2').versionRef('lib2-version')
    }
}

---

toml:
[versions]
lib2-version = ""

[libraries]
lib1 = {group = "my", name = "library1", version = "1.0.0" }
lib2 = {group = "my", name = "library2", version.ref = "lib2-version" }

---

settings.gradle:
dependencyResolutionManagement {
    versionCatalogs {
        libs {
            from("my:catalog:1.0.0")
            // version('lib2-version', '1.5.0')
        }
    }
}

I'd expect that this should work in a very similar fashion as in the first case. After all, the version is "" in both cases. But it does not and fails with

Invalid TOML catalog definition:
- Problem: In version catalog libs, alias definition 'lib2-version' is invalid.    
  Reason: Version for 'lib2-version' must not be empty.  
  Possible solution: Set a value for 'version'.

Which feels like an artificial restriction.