Enhance null handling in switch transformations

- Updated `Issue3421.cs`.
- Updated `MatchLegacySwitchOnStringWithDict` to check for `leaveContainer` and handle null sections accordingly.
- Introduced an overload for `AddNullSection` to accept `ILInstruction` as the body, improving flexibility.
- Modified existing `AddNullSection` to utilize the new overload, allowing for varied body types in `SwitchSection`.

ICSharpCode.Decompiler.Tests/TestCases/ILPretty/Issue3421.cs

@@ -5,10 +5,6 @@ internal class Issue3421
 
 	public virtual void SetValue(object value)
 	{
-		if (name == null)
-		{
-			return;
-		}
 		switch (name)
 		{
 			case "##Name##":
@@ -19,6 +15,8 @@ internal class Issue3421
 			case "##InnerText##":
 				this.value = value.ToString();
 				return;
+			case null:
+				return;
 		}
 		if (this.value == null)
 		{

ICSharpCode.Decompiler/IL/Transforms/SwitchOnStringTransform.cs

@@ -675,6 +675,13 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 					return false;
 				}
 			}
+			else if (leaveContainer != null && !defaultBlockJump.MatchLeave(leaveContainer))
+			{
+				if (!AddNullSection(sections, stringValues, (Leave)exitBlockJump))
+				{
+					return false;
+				}
+			}
 			context.Step(nameof(MatchLegacySwitchOnStringWithDict), instructions[i]);
 			bool keepAssignmentBefore = false;
 			if (switchValueVar.LoadCount > 2 || switchValue == null)
@@ -741,6 +748,11 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 		}
 
 		bool AddNullSection(List<SwitchSection> sections, List<(string Value, int Index)> stringValues, Block nullValueCaseBlock)
+		{
+			return AddNullSection(sections, stringValues, new Branch(nullValueCaseBlock));
+		}
+
+		bool AddNullSection(List<SwitchSection> sections, List<(string Value, int Index)> stringValues, ILInstruction body)
 		{
 			var label = new LongSet(stringValues.Max(item => item.Index) + 1);
 			var possibleConflicts = sections.Where(sec => sec.Labels.Overlaps(label)).ToArray();
@@ -753,7 +765,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 				possibleConflicts[0].Labels = possibleConflicts[0].Labels.ExceptWith(label);
 			}
 			stringValues.Add((null, (int)label.Values.First()));
-			sections.Add(new SwitchSection() { Labels = label, Body = new Branch(nullValueCaseBlock) });
+			sections.Add(new SwitchSection() { Labels = label, Body = body });
 			return true;
 		}