fix mermaid with one job (in a flow)

[JaGeo]

Summary

Closes the issue described in #349 . As described there, I am not sure if I missed a use-case for the following line

jobflow/src/jobflow/utils/graph.py

Line 246 in 09ffa16

if indent_level != 1:

I did not see any change in other tested workflow visualizations.

[utf]

Thanks! Can't remember why I added that line.