Convert more undefined variables to default value at lock time

[Alizter]

We should be catching more undefined variables than we currently do. For example %{pkg:dkml-base-compiler:version} should be an empty string according to opam semantics. Furthermore, various installed variables can be set to either the empty string (in a string context) or false in a boolean context. This should be done at lock time and used to simplify the expression.

• installed
  • fix(pkg): substitute installed variable at solve time #13776
• version
  • fix(pkg): substitute version variable in string interpolation #13785
• others (fill in as appropriate)