Minor wagl fixes

[jeremyh]

A few more fixes that have caused some minor failures.

• When no tesp doc was found by the wrapper, a variable was being referenced without initialisation, causing a python error.
• NaN cloud values shouldn't override existing numbers. This NaNs would cause failures on a few datasets in downstream processing.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (17f80b6) 84.77% compared to head (0bcbff5) 84.78%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #300   +/-   ##
========================================
  Coverage    84.77%   84.78%           
========================================
  Files           26       26           
  Lines         4408     4410    +2     
========================================
+ Hits          3737     3739    +2     
  Misses         671      671           

Impacted Files	Coverage Δ
eodatasets3/wagl.py	88.39% <100.00%> (+0.06%)	⬆️

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