[DependencyInjection] GraphvizDumper now displays unresolved parameters

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | none
| License       | MIT
| Doc PR        | none

Previously, the graphviz dumper would throw an exception if given a container with an unresolved parameter. This change allows it to render the parameter name instead of the class name if the class can not be resolved. This is also closer to the other DI dumpers which do not error out in these circumstances but simply carry the parameter name over.

For a practical use case, see: https://github.com/rosstuck/TuckConverterBundle/issues/1