Fix manufacturer string when terminating with both `\x00` and `\xFF`

[puddly]

The manufacturer string in some boards is encoded strangely:

ffffffffffffffffffffffffffff0000

The null byte terminator should be used when encoding a valid string. This one, however, is not.

Instead of reading out 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFF we should return None.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.16%. Comparing base (c4578a0) to head (1464e7a).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #638 +/- ## ======================================= Coverage 99.16% 99.16% ======================================= Files 50 50 Lines 3459 3460 +1 ======================================= + Hits 3430 3431 +1 Misses 29 29 ```

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