Unknown flag's diagnostic may be of length zero

[rcjsuen]

FROM node
HEALTHCHECK -- CMD ls
COPY -- . .
ADD -- . .

If a flag is present but it doesn't actually have a name, a diagnostic will be created to warn about the invalid flag. However, as it has no name, the range will incorrectly be zero which can lead to conflicting behaviours in clients regarding how to highlight the error. For example, the monaco-languageclient seems to highlight the space after the range which may be confusing to the user as it is highlighting whitespace as an error.