For empty format strings, default to built-in .NET formatter

[igitur]

Currently, if you pass an empty format string, an exception is raised. I would expect that some default format would be used. I don't think this should necessarily be what Excel would use as a default. I think the build-in .NET ToString() method is just fine, but you'll have to say whether this fits in with what you expect.

So this PR:

• returns default .NET format of value
• reworks the logic a bit from multiple if statements to a switch statement.