Minor corrections to unit tests

[gavinevans]

Addresses metoppv#2135

Description
This PR adds the catching of an exception that can occur if the removal of rows with NaNs from the truth DataFrame subsequently means that there are no matching rows between the forecast and truth DataFrames, in terms of the site_id (e.g. wmo_id) and time. Minor edits to the PrepareAndTrainQRF unit tests are also made.

Testing:

• Ran tests and they passed OK
• Added new tests for the new feature(s)

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (mobt_877_implement_qrf@38fb827). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...oad_and_train_quantile_regression_random_forest.py	0.00%	3 Missing ⚠️
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Additional details and impacted files

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##             mobt_877_implement_qrf      #22   +/-   ##
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  Misses                            ?      626           
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