Issue8 fix

[Dunkhan]

Fixes #8

The issue was that empty and null strings were being measured, returning the font height and 0 width. This was creating additional space wherever a text could be shown but was not.

The behaviour implemented in this PR is copied directly from the original oxyplot CanvasRenderContext https://github.com/oxyplot/oxyplot/blob/fe84b034c1105fd0cd679d8cedd88d9c20fde417/Source/OxyPlot.Wpf/CanvasRenderContext.cs and simply returns OxySize.Empty for null and empty strings

Checklist

• [ ] I have included examples or tests
• [x] I have updated the change log
• [ ] I am listed in the CONTRIBUTORS file
• [x] I have cleaned up the commit history (use rebase and squash)

@oxyplot/admins

[objorke]

thanks!