TransformExpressionTrees: do not crash on a lambda that did not convert

The builder returned by ConvertLambda hands back null when a nested
conversion declines, and the result was cast and dereferenced before
anything checked it, so a tree the transform cannot handle took down the
whole method with a NullReferenceException instead of being left alone.

EF Core's StringCharConverter.ToChar is such a tree: the conditional
spills the Expression.Call arguments into stack slots, which
MatchGetMethodFromHandle does not see through.

Assisted-by: Claude:claude-opus-5:Claude Code

ICSharpCode.Decompiler/IL/Transforms/TransformExpressionTrees.cs

@@ -171,8 +171,11 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 				if (lambda != null)
 				{
 					context.Step("Convert Expression Tree", instruction);
-					var newLambda = (ILFunction)lambda();
-					if (newLambda == null)
+					// A builder returns null where a nested conversion declined; the cast has to
+					// tolerate that, and anything that is not an ILFunction is no lambda to put
+					// in the call's place either. The tree then stays as the Expression calls
+					// that built it.
+					if (lambda() is not ILFunction newLambda)
 						return false;
 					SetExpressionTreeFlag(newLambda, (CallInstruction)instruction);
 					instruction.ReplaceWith(newLambda);