Warn on empty string

[cmoesel]

FHIR indicates that string values should have at least one non-whitespace character (see: https://build.fhir.org/datatypes#string). SUSHI should warn users when they assign an empty string.

If this is complex to do for arbitrary assignments, we should at least do it for the major metadata keywords (Title:, Description:, etc) -- especially since the VS Code snippets produce empty strings for these by default.

See: https://chat.fhir.org/#narrow/stream/179252-IG-creation/topic/Miss.20leading.20QA.2Ehtml.20results/near/282548095