Fix bug when processDocument() is run on a single word

[stephaniereinders]

doc <- handwriter::processDocument('path/to/image.png')

In the result, doc$process$letterList isn't in the correct format. letterList should have 2 items, one for each graph. Each item should have a nested list of 8 fields. Instead, doc$process$letterList is a list of 16 items.

[stephaniereinders]

The changes fixed the error for 26 docs, each containing the word "the", from CSAFE writer 1, but the error occurs for docs from CSAFE writer 2.