Increase coverage.

Author: sharpjs

PSql.Engine.Tests/SqlCmdPreprocessorTests.cs

@@ -52,6 +52,12 @@ public void SetVariables_EntryHasEmptyKey()
         TestSetVariablesWith(entry: new DictionaryEntry("", "value"));
     }
 
+    [Test]
+    public void SetVariables_EntryHasNullValue()
+    {
+        TestSetVariablesWith(entry: new DictionaryEntry("a", null));
+    }
+
     private static void TestSetVariablesWith(object entry)
     {
         var preprocessor = new SqlCmdPreprocessor();

PSql.Engine/SqlCmdPreprocessor.cs

@@ -34,7 +34,7 @@ public bool EnableVariableReplacementInComments
     /// </param>
     public void SetVariables(IDictionary? entries)
     {
-        if (entries == null)
+        if (entries is null)
             return;
 
         var variables = _preprocessor.Variables;