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[bot]]

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

```diff @@ 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](https://codecov.io/gh/opendatacube/eo-datasets/pull/300?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=opendatacube) | Coverage Δ | | |---|---|---| | [eodatasets3/wagl.py](https://codecov.io/gh/opendatacube/eo-datasets/pull/300?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=opendatacube#diff-ZW9kYXRhc2V0czMvd2FnbC5weQ==) | `88.39% <100.00%> (+0.06%)` | :arrow_up: |

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