Normalize scrubbed-to-empty text to emptyString

src/Verify.Tests/Converters/ExtensionConverterTests.NestedTarget#03.verified.txt

@@ -1 +1 @@
-
+emptyString

src/Verify.Tests/LinesScrubberTests.ScrubAllLinesToEmpty.verified.txt

@@ -0,0 +1 @@
+emptyString

src/Verify.Tests/LinesScrubberTests.cs

@@ -61,6 +61,23 @@ public Task FilterLines()
                     """);
     }
 
+    [Fact]
+    public Task ScrubAllLinesToEmpty()
+    {
+        // Scrubbing away every line leaves empty content. It is normalized to the
+        // "emptyString" sentinel (the same one used for Verify(string.Empty)) so that
+        // empty content is never written as a snapshot nor passed to a string comparer.
+        var settings = new VerifySettings();
+        settings.ScrubLines(removeLine: _ => true);
+        return Verify(
+            settings: settings,
+            target: """
+                    a
+                    b
+                    c
+                    """);
+    }
+
     [Fact]
     public Task ScrubLinesContaining_case_sensitive()
     {

src/Verify/Verifier/InnerVerifier_Inner.cs

@@ -136,6 +136,14 @@ void Scrub(in Target target)
         if (target.TryGetStringBuilder(out var builder))
         {
             ApplyScrubbers.ApplyForExtension(target.Extension, builder, settings, counter);
+            // Content scrubbed away entirely is normalized to the same sentinel used for an
+            // empty root string (see TryGetRootTarget). This keeps empty out of the written
+            // snapshot and, more importantly, out of any registered string comparer, which may
+            // assume non-empty input.
+            if (builder.Length == 0)
+            {
+                builder.Append("emptyString");
+            }
         }
     }
 
@@ -170,9 +178,9 @@ bool TryGetRootTarget(object? root,bool ignoreNullRoot, [NotNullWhen(true)] out
             }
             else
             {
-                var builder = new StringBuilder(stringRoot);
-                ApplyScrubbers.ApplyForExtension("txt", builder, settings, counter);
-                target = new("txt", builder);
+                var textTarget = new Target("txt", new StringBuilder(stringRoot));
+                Scrub(textTarget);
+                target = textTarget;
             }
 
             return true;