Handling Nulls in Currency Filter (#477)

src/DotLiquid.Tests/StandardFilterTests.cs

@@ -714,6 +714,26 @@ public void TestEuroCurrencyFromString(object input, string expected, string lan
             Assert.AreEqual(expected, StandardFilters.Currency(context: _contextV20, input: input, languageTag: languageTag));
         }
 
+        [Test]
+        [TestCase(null)]
+        [TestCase("")]
+        public void TestNullOrEmptyInputCurrency(string input)
+        {
+            // Set the thread culture and test for backward compatibility
+            using (CultureHelper.SetCulture("en-US"))
+            {
+                Helper.AssertTemplateResult(
+                    expected: string.Empty,
+                    template: "{{ input | currency: 'de-DE' }}",
+                    localVariables: Hash.FromAnonymousObject(new { input = input }));
+            }
+
+            // _contextV20 is initialized with InvariantCulture
+            Assert.AreEqual(
+                expected: input,
+                actual: StandardFilters.Currency(context: _contextV20, input: input, languageTag: "de-DE"));
+        }        
+
         [Test]
         public void TestMalformedCurrency()
         {

src/DotLiquid/StandardFilters.cs

@@ -359,6 +359,9 @@ public static string Rstrip(string input)
         /// <seealso href="https://shopify.dev/api/liquid/filters/money-filters#money">Shopify Money filter</seealso>
         public static string Currency(Context context, object input, string languageTag = null)
         {
+            // Check for null input, return null
+            if (input == null) return null;
+
             // Check for null only, allow an empty string as it represent the InvariantCulture
             var culture = languageTag == null ? context.CurrentCulture : new CultureInfo(languageTag.Trim());