GEOPY-2619: Joint surveys fails validation if no starting/reference models chosen #1418

Job	Run time
Static analysis / pylint (ubuntu-latest)	1m 53s
Pytest / Pytest (3.10, ubuntu-latest)	8m 27s
Pytest / Pytest (3.12, windows-latest)	13m 19s
Pytest / Pytest (3.10, windows-latest)	13m 33s
Pytest / Pytest (3.11, windows-latest)	13m 28s
Pytest / Pytest (3.11, ubuntu-latest)	7m 35s
Pytest / Pytest (3.12, ubuntu-latest)	8m 34s
	1h 6m 49s

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