Fix error on create_vanadium if no sample details in ISIS powder reduction

[RichardWaiteSTFC]

Description of work

Fix error on create_vanadium if no sample details in ISIS powder reduction - was checking if the sample details had been set on the class, but the sample details for vanadium are set in the advanced config automatically.

Purpose of work

Bug report from Helen Playford (POLARIS, ISIS) - was getting ValueError when calling create_vanadium and no sample details set.

It seems to have been introduced in this PR #36809 in v6.11

There is no associated issue.

Report to: Helen Playford (POLARIS, ISIS)

To test:

(1) Ask me for the config files

(2) Run this script with updated paths

from isis_powder import polaris, SampleDetails

config_file_path = r"C:\Users\xhg73778\Downloads\POLARISPowderDiffractionFolderSmall\polaris_config_example.yaml"
polaris = polaris.Polaris(config_file=config_file_path, user_name="test", mode="PDF")

polaris.create_vanadium(first_cycle_run_no="98533", multiple_scattering=True, 
                         do_absorb_corrections=True, mayers_mult_scat_events=1)   

Should execute without error.

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[GuiMacielPereira]

Ran the script without problems. Unit tests added check for both vanadium present and not present.