fix: nullReferenceException thrown when FileLinePositionSpan does not…

… contain a path (#289)

src/Tests/Tests.FileLinePositionSpanConverter_Empty.verified.txt

@@ -0,0 +1 @@
+

src/Tests/Tests.cs

@@ -21,6 +21,10 @@ public Task LocalizableStringConverter() =>
     public Task FileLinePositionSpanConverter() =>
         Verify(new FileLinePositionSpan("the path", new(1,2), new(2,4)));
 
+    [Fact]
+    public Task FileLinePositionSpanConverter_Empty() =>
+        Verify(new FileLinePositionSpan());
+
     class LocalizableStringImp :
         LocalizableString
     {

src/Verify.SourceGenerators/Converters/FileLinePositionSpanConverter.cs

@@ -1,6 +1,11 @@
 class FileLinePositionSpanConverter :
     WriteOnlyJsonConverter<FileLinePositionSpan>
 {
-    public override void Write(VerifyJsonWriter writer, FileLinePositionSpan value) =>
-        writer.WriteValue($"{value.Path.Replace('/', '\\')}: {value.Span}");
+    public override void Write(VerifyJsonWriter writer, FileLinePositionSpan value)
+    {
+        if (value.IsValid)
+        {
+            writer.WriteValue($"{value.Path.Replace('/', '\\')}: {value.Span}");
+        }
+    }
 }