fix: adds error message for empty SDL

[EverlastingBugstopper]

fixes #724

before:

$ rover subgraph publish my-graph --routing-url https://example.com --schema reviews.graphql --name reviews --convert
Publishing SDL to my-graph@current (subgraph: reviews) using credentials from the default profile.
error: Partial schema input must have at least one of "sdl" or "hash"

after:

$ rover subgraph publish my-graph --routing-url https://example.com --schema reviews.graphql --name reviews --convert
Publishing SDL to my-graph@current (subgraph: reviews) using credentials from the default profile.
error: The provided SDL cannot be an empty string.