Fix definite assignment error in regex source gen for some lazy loops (#104007)

Author: stephentoub

src/libraries/System.Text.RegularExpressions/gen/RegexGenerator.Emitter.cs

@@ -3458,7 +3458,8 @@ void EmitSingleCharLazy(RegexNode node, RegexNode? subsequent = null, bool emitL
                     maxIterations = $"{node.N - node.M}";
 
                     iterationCount = ReserveName("lazyloop_iteration");
-                    writer.WriteLine($"int {iterationCount} = 0;");
+                    additionalDeclarations.Add($"int {iterationCount} = 0;");
+                    writer.WriteLine($"{iterationCount} = 0;");
                 }
 
                 // Track the current crawl position.  Upon backtracking, we'll unwind any captures beyond this point.

src/libraries/System.Text.RegularExpressions/tests/FunctionalTests/Regex.Groups.Tests.cs

@@ -956,6 +956,8 @@ public static IEnumerable<object[]> Groups_MemberData()
 
                 yield return (enUS, @"(?((\w{3}))\1\1|no)", "dogdogdog", RegexOptions.None, new string[] { "dogdog", "dog" });
                 yield return (enUS, @"(?((\w{3}))\1\1|no)", "no", RegexOptions.None, new string[] { "no", "" });
+
+                yield return (enUS, @"(?()|.??(?<=))(?!.)", "", RegexOptions.None, new string[] { "" });
             }
 
             // Special cases involving starting position search optimizations