MappingKernelManager.start_kernel should use kernel_id for new kernel if it doesn't exist

[vidartf]

The docstring of MappingKernelManager.start_kernel says:

jupyter_server/jupyter_server/services/kernels/kernelmanager.py

Lines 170 to 173 in 122d93a

	kernel_id : uuid
	The uuid to associate the new kernel with. If this
	is not None, this kernel will be persistent whenever it is
	requested.

But if the kernel id doesn't currently exist, it throws an error:

jupyter_server/jupyter_server/services/kernels/kernelmanager.py

Line 202 in 122d93a

self._check_kernel_id(kernel_id)

So, the method should probably be changed to reflect the docstring (which is consistent with the behavior in jupyter client).

[kevin-bates]

Boy, this has been this way for years, but, yeah, it looks like the if kernel_id is None: should be extended with or kernel_id not in self and reflected in the args of the call to the subclass.

Good catch.