Fix line break enumerator infinite loop (#21808)

* Add failing test: LineBreakEnumerator infinite loop on empty string

* Return false immediately for empty string in LineBreakEnumerator

Empty text has no content and should not produce any line break
opportunities. Short-circuit in MoveNext before entering the
read/rule loop, while keeping the PeekAt EndOfText fix as a
defensive safeguard.

src/Avalonia.Base/Media/TextFormatting/Unicode/LineBreakEnumerator.cs

@@ -24,6 +24,11 @@ namespace Avalonia.Media.TextFormatting.Unicode
         {
             lineBreak = default;
 
+            if (_text.IsEmpty)
+            {
+                return false;
+            }
+
             if (_state.Current.EndOfText)
             {
                 return false;

tests/Avalonia.Base.UnitTests/Media/TextFormatting/LineBreakEnumeratorTests.cs

@@ -19,6 +19,14 @@ namespace Avalonia.Base.UnitTests.Media.TextFormatting
             _outputHelper = outputHelper;
         }
 
+        [Fact]
+        public void ShouldHandleEmptyString()
+        {
+            var lineBreaker = new LineBreakEnumerator(string.Empty);
+
+            Assert.False(lineBreaker.MoveNext(out _));
+        }
+
         [Fact]
         public void BasicLatinTest()
         {