Avoid null args when RazorDiagnostic does not have a message (#37648)

* Avoid null args when RazorDiagnostic does not have a message

We've received tooling reports of the Razor compiler failing with a argument null exception
when trying to format a diagnostic message. Unfortunately dumps don't have enough details to indicate
what code path causes the badly formated argument to be used. Additonally, inspecting the code and turning
on nullability in this code path does not suggest any obvious candidates in Razor's codebase.

Tooling is working around this by try-catching the exception, but this results in squiggles / error items that do not
disappear. This PR introduces a temporary workaround that prints a generic error message so that it does not crash VS
/ require tooling workarounds while we try and figure out the root cause.

Fixes https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1405849

src/Razor/Microsoft.AspNetCore.Razor.Language/src/RazorDiagnosticDescriptor.cs

-// Licensed to the .NET Foundation under one or more agreements.
+// Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System;
@@ -35,7 +35,16 @@ namespace Microsoft.AspNetCore.Razor.Language
 
         public RazorDiagnosticSeverity Severity { get; }
 
-        public string GetMessageFormat() => _messageFormat();
+        public string GetMessageFormat()
+        {
+            var message = _messageFormat();
+            if (string.IsNullOrEmpty(message))
+            {
+                return Resources.FormatRazorDiagnosticDescriptor_DefaultError(Id);
+            }
+
+            return message;
+        }
 
         public override bool Equals(object obj)
         {

src/Razor/Microsoft.AspNetCore.Razor.Language/src/Resources.resx

@@ -574,4 +574,7 @@
   <data name="Component_EditorRequiredParameterNotSpecified" xml:space="preserve">
     <value>Component '{0}' expects a value for the parameter '{1}', but a value may not have been provided.</value>
   </data>
-</root>
+  <data name="RazorDiagnosticDescriptor_DefaultError" xml:space="preserve">
+    <value>Encountered diagnostic '{0}'.</value>
+  </data>
+</root>
\ No newline at end of file

src/Razor/Microsoft.AspNetCore.Razor.Language/test/RazorDiagnosticDescriptorTest.cs

-// Licensed to the .NET Foundation under one or more agreements.
+// Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using Xunit;
@@ -74,5 +74,17 @@ namespace Microsoft.AspNetCore.Razor.Language
             // Assert
             Assert.False(result);
         }
+
+        [Fact]
+        public void RazorDiagnosticDescriptor_NullMessage()
+        {
+            // Arrange & Act
+            var descriptor = new RazorDiagnosticDescriptor("RZ0001", () => null, RazorDiagnosticSeverity.Error);
+
+            // Assert
+            Assert.Equal("RZ0001", descriptor.Id);
+            Assert.Equal(RazorDiagnosticSeverity.Error, descriptor.Severity);
+            Assert.Equal("Encountered diagnostic 'RZ0001'.", descriptor.GetMessageFormat());
+        }
     }
 }

src/Razor/Microsoft.AspNetCore.Razor.Language/test/RazorDiagnosticTest.cs

-// Licensed to the .NET Foundation under one or more agreements.
+// Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Globalization;
 using Microsoft.AspNetCore.Razor.Language.Legacy;
 using Xunit;
 
@@ -41,5 +42,20 @@ namespace Microsoft.AspNetCore.Razor.Language
             Assert.Equal(RazorDiagnosticSeverity.Error, defaultDiagnostic.Severity);
             Assert.Equal(span, diagnostic.Span);
         }
+
+        [Fact]
+        public void GetMessage_WithNullDescriptorFormat_ReturnsDefaultErrorString()
+        {
+            // Arrange
+            var descriptor = new RazorDiagnosticDescriptor("RZ0001", () => null, RazorDiagnosticSeverity.Error);
+            var span = new SourceSpan("test.cs", 15, 1, 8, 5);
+
+            // Act
+            var diagnostic = RazorDiagnostic.Create(descriptor, span, "Hello", "World");
+            var message = diagnostic.GetMessage(CultureInfo.InvariantCulture);
+
+            // Assert
+            Assert.Equal("Encountered diagnostic 'RZ0001'.", message);
+        }
     }
 }