YAMS decodes empty string as nil

[tjprescott]

jpsim/Yams#253

When YAMS encounters an empty string when decoding a YAML string to Swift objects, it decodes it as nil rather than as an empty string. This results in discrepancies between the original file and the round-tripped file.

As a workaround, replace all empty strings in the code model file with a space '' -> ' '

[tjprescott]

Workaround has been implemented. Leaving this issue open for tracking.