Fix NRE when creating null StringValue

Fluid.Tests/StringValueTests.cs

@@ -18,5 +18,14 @@ public void CreateStringValue(string value, string expected, bool isEmpty)
             Assert.Equal(expected, stringValue.ToStringValue());
             Assert.Equal(isEmpty, stringValue.Equals(StringValue.Empty));
         }
+
+        [Fact]
+        public void StringValueCreateNullShouldReturnEmpty()
+        {
+            var stringValue = new StringValue(null);
+
+            // Assert
+            Assert.Equal(StringValue.Empty, stringValue);
+        }
     }
 }

Fluid/Values/StringValue.cs

@@ -1,10 +1,10 @@
-using System;
+using Parlot;
+using System;
 using System.Collections.Generic;
 using System.Globalization;
 using System.IO;
 using System.Runtime.CompilerServices;
 using System.Text.Encodings.Web;
-using Parlot;
 
 namespace Fluid.Values
 {
@@ -27,6 +27,9 @@ static StringValue()
 
         public StringValue(string value)
         {
+            // Returns a StringValue instance and not NilValue since this is what is asked for.
+            // However FluidValue.Create(null) returns NilValue.
+
             _value = value ?? NilValue.Instance.ToStringValue();
         }
 
@@ -56,7 +59,7 @@ internal static StringValue Create(char c)
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public static StringValue Create(string s)
         {
-            if (s == "")
+            if (String.IsNullOrEmpty(s))
             {
                 return Empty;
             }
@@ -66,6 +69,12 @@ public static StringValue Create(string s)
                 : new StringValue(s);
         }
 
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static StringValue Create(string s, bool encode)
+        {
+            return Create(s, encode);
+        }
+
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         internal static StringValue Create(in TextSpan span)
         {