arrow-pg: panic when encoding dictionary arrays

[savani02]

I noticed that there was a small bug with how the DataType::Dictionary branch retrieves values - at the moment it uses the row index to directly access the dictionary's values array which means that in circumstances where the row index is greater than the length of the dictionary, we hit an out of bounds error.

A quick fix for this could be to access the keys first, get the row index and use that instead to extract values. We could also add a test to just check that this works and hits the happy path.

[sunng87]

@savani02 Thank you! A pull request is welcomed.

[savani02]

Sure! Here's the pr: fix: resolve arrow-pg panic when encoding dictionary arrays