throw exception instead of null if wrong type in BorderValue

[freemansoft]

We want to throw if an invalid token definition was used which would be anything other than a map of the 3 attributes of "value".

1. Border value returns null instead of throwing an exception if the value is anything other than a Map<String, ...>
2. Retains the current behavior of returning null if null was passed in.

This causes a hard to debug later when a null check catches the null.

This code instead throws a FormatException that will get later wrapped and logged making debugging easier.

[codecov[bot]]

Codecov Report

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

Project coverage is 98.46%. Comparing base (b20e952) to head (817346c).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #22 +/- ## ======================================= Coverage 98.46% 98.46% ======================================= Files 31 31 Lines 847 849 +2 ======================================= + Hits 834 836 +2 Misses 13 13 ```

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[freemansoft]

The previous unit test saved me from doing something stupid. Let's hear it for regression protection tests!