Some clients have been explicitly checking whether the string specified
after a keyword is empty. Offer them a way to specify that the string
must be non-empty as part of the binding type.
Bindings to `std::string` require one value. Some clients have been
filtering `keywordsMissingValue` to support keywords that tolerate a
missing value. Offer them a type-safe way to achieve this instead.
Previously bindings to `std::vector<std::string>` required at least one
value. Some clients have been filtering `keywordsMissingValue` to
support keywords followed by empty lists. Instead, require clients to
specify whether a keyword's list can be empty as part of the binding
type.
