handle missing and invalid values when prompting

[lkingland]

When a user provides an invalid option for a flag, a prompt selection default is not actually selected despite its UX showing selected.

When a flag or environment variable being used as a prompt selection is empty (has no default value), the selection default is not actually selected despite its UX showing selected.

In situations where a Flag is being used to directly set the default value for a survey.Select prompt, that default should only be set when the value provided by the user both exists and is one of the options.

[lkingland]

See #814 which illustrates the problem clearly. See #815 for a first-pass solution.