Handle empty package namespace in package selectors

[mcombuechen]

Closes #85.

Solutions to the problem:

• fall back on a non-breaking space when a namespace option label is empty
• make the state values of package type, namespace, name nullable to better handle an unselected state

[mcombuechen]

Some screenshots

a. The empty namespace option now renders more nicely:

b. When selecting the empty namespace option, the package name selector gets enabled.

[mcombuechen]

oops I see now this is partly a duplicate of #77