fix: disallow undefined xml

[mshanemc]

What does this PR do?

existing code was allowing xml: undefined to match the SourceComponent type, but it doesn't really. Then other code that expected a SC would throw unexpectedly

What issues does this PR fix or reference?

found during QA of forcedotcom/source-deploy-retrieve#1093

@W-14138121@

[git2gus]

This issue has been linked to a new work item: W-14138121

[shetzel]

Worth adding a unit test?

[cristiand391]

QA notes:

before:

➜  dreamhouse-lwc git:(main) ✗ cat .forceignore
# First, ignore everything
*
➜  dreamhouse-lwc git:(main) ✗ sf project deploy preview -o dreamhouse
Error (1): The "path" argument must be of type string. Received undefined

with this fix:

➜  dreamhouse-lwc git:(main) ✗ cat .forceignore
# First, ignore everything
*
➜  dreamhouse-lwc git:(main) ✗ sf project deploy preview -o dreamhouse

No conflicts found.

No files will be deleted.

No files will be deployed.

No files were ignored. Update your .forceignore file if you want to ignore certain files.