handle `package-version` safely to prevent `listp` error

[yaaama]

When package-version' is not what we expect (e.g a simple string), a listp` error is thrown and you are unable to view the variable documentation due to it. This now handles it and provides a fallback value.

This was a recurring problem I was facing when opening which-key variables using helpful. which-key would send a value of "31.0" and then cause this error:

helpful--version-info: Wrong type argument: listp, "1.0"