Handle missing caller identifier

Fixes #3031
Regression from #3000 where previously `getCallerIdentifier() + callerPostfix` would return an empty string.

Author: jnyrup

Src/FluentAssertions/Execution/SubjectIdentificationBuilder.cs

@@ -83,7 +83,7 @@ public string Build()
 
         if (scopeName is null)
         {
-            return callerIdentifier;
+            return callerIdentifier ?? "";
         }
         else if (callerIdentifier is null)
         {

Tests/FluentAssertions.Specs/Execution/AssertionChainSpecs.Chaining.cs

@@ -578,6 +578,20 @@ public void Discard_a_scope_after_continuing_chained_assertion()
             Assert.Contains("First \"assertion\"", failures);
         }
 
+        [Fact]
+        public void Returns_an_empty_identification_when_neither_scope_name_nor_caller_identifier_are_available()
+        {
+            // Arrange
+            var assertionChain = AssertionChain.GetOrCreate();
+            assertionChain.OverrideCallerIdentifier(() => null);
+
+            // Act
+            string identification = assertionChain.CallerIdentifier;
+
+            // Assert
+            identification.Should().BeEmpty();
+        }
+
         // [Fact]
         // public void Get_info_about_line_breaks_from_parent_scope_after_continuing_chained_assertion()
         // {

docs/_pages/releases.md

@@ -7,6 +7,12 @@ sidebar:
   nav: "sidebar"
 ---
 
+## 8.1.2
+
+## Fixes
+
+* Fixed a regression from 8.1.0 where a `NullReferenceException` was thrown during subject identification - [#3036](https://github.com/fluentassertions/fluentassertions/pull/3036
+
 ## 8.1.1
 
 ## Fixes