Crash when mutated code contains partial method declaration

[oliwennell]

This line in CoverageAnalyser.cs:

var firstChildNode = methodNode.Body.ChildNodes().FirstOrDefault();

...throws a NullReferenceException when .Body is null, causing Fettle to quit.

.Body will be null when a method doesn't have a "normal" body. Fettle already handles this for expression-bodied methods and abstract methods, but not partial method declarations. E.g.

public abstract void Thing();