Jupyter: Only pass the --LabApp.default_url flag if we have something… #25

Job	Run time
discover-tutorials	8s
build-and-push (tutorials/stdpar)	26m 34s
build-and-push (tutorials/cuda-cpp)	8m 55s
build-and-push (tutorials/accelerated-python)	14m 45s
	50m 22s

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