Validate creating a branch when source has no instantiated graph

[dlamoris]

In BranchWrite, if the source has no instantiated graph available, it creates a lock and a Model graph instead of a new branch and staging graph.

This condition currently never happens because a commit lock is created after each commit, so on creating a branch or lock there's always a graph to copy from.

Relevant code:
case where it creates a branch staging graph: https://github.com/Open-MBEE/flexo-mms-layer1-service/blob/develop/src/main/kotlin/org/openmbee/flexo/mms/routes/ldp/BranchWrite.kt#L255
case where it creates a lock if there's no existing graph to copy: https://github.com/Open-MBEE/flexo-mms-layer1-service/blob/develop/src/main/kotlin/org/openmbee/flexo/mms/routes/ldp/BranchWrite.kt#L371

related to #86