Handle icons for None sensor values gracefully of huawei_lte

[scop]

Description:

Avoid errors when trying to render icons for None values.

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.

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If the code does not interact with devices:

• Tests have been added to verify that the new code works.