Various compatibility fixes (#7)

Substring length can be longer than the end of string, when that happens, go to the end of the string.

TREATASCONTENT may have empty values and that's OK.

IF statements on empty values should return false.

IF statements without clauses should check for equal to true

src/Sage.Engine.Tests/Corpus/Function/substring.txt

@@ -47,3 +47,12 @@ set @RESULT = Substring('Logan',1.5)
 %%=v(@RESULT)=%%
 ++++++++++
 !
+==========
+Substring('Logan',1,50)
+==========
+%%[
+set @RESULT = Substring('Logan',1,50)
+]%%
+%%=v(@RESULT)=%%
+++++++++++
+Logan

src/Sage.Engine.Tests/Corpus/Function/treatascontent.txt

@@ -9,4 +9,9 @@ TREATASCONTENTAREA
 ==========
 %%= TREATASCONTENTAREA("Content",CONCAT("%", "%", "=ADD(1,2)=", "%", "%")) =%%
 ++++++++++
-3
+3
+==========
+Empty TREATASCONTENT
+==========
+%%=TREATASCONTENT("")=%%
+++++++++++

src/Sage.Engine.Tests/Corpus/Language/if.txt

@@ -0,0 +1,15 @@
+===========
+NULL IF CHECK
+===========
+%%[
+	VAR @variable
+
+    IF @variable THEN
+        OUTPUT("1")
+    ENDIF
+    IF NOT @variable THEN
+        OUTPUT("2")
+    ENDIF
+]%%
+++++++++++
+2

src/Sage.Engine.Tests/Sage.Engine.Tests.csproj

@@ -81,6 +81,9 @@
     <None Update="Corpus\Language\assignment.txt">
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
     </None>
+    <None Update="Corpus\Language\if.txt">
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+    </None>
     <None Update="Corpus\Language\for.txt">
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
     </None>

src/Sage.Engine/Runtime/Functions/Content.cs

@@ -171,6 +171,11 @@ public string CONTENTBLOCKBYKEY(object contentBlockKey, object? impressionRegion
         public string TREATASCONTENT(object? content)
         {
             string contentString = content?.ToString() ?? string.Empty;
+
+            if (string.IsNullOrWhiteSpace(contentString))
+            {
+                return contentString;
+            }
 
             return CompileAndExecuteEmbeddedCodeAsync($"treatascontent__{_stackFrame.Peek().CurrentLineNumber}", contentString) ?? string.Empty;
         }

src/Sage.Engine/Runtime/Functions/String.cs

@@ -150,7 +150,19 @@ public string SUBSTRING(object subject, object start, object? length = null)
 
             int lengthInt = SageValue.ToInt(length);
 
-            return subject?.ToString()?.Substring(startInt - 1, lengthInt) ?? string.Empty;
+            string subjectToCheck = subject?.ToString() ?? string.Empty;
+
+            if (string.IsNullOrEmpty(subjectToCheck))
+            {
+                return string.Empty;
+            }
+
+            if (lengthInt > subjectToCheck.Length - startInt)
+            {
+                lengthInt = (subjectToCheck.Length - startInt + 1);
+            }
+
+            return subjectToCheck.Substring(startInt - 1, lengthInt);
         }
 
         /// <summary>

src/Sage.Engine/Runtime/SageValue.cs

@@ -382,6 +382,12 @@ public static DateTimeOffset ToDateTime(object? inputObj, IFormatProvider? forma
         public static UnboxResult TryToBoolean(object? inputObj, out bool result)
         {
             inputObj = UnboxVariable(inputObj);
+
+            if (inputObj == null)
+            {
+                result = false;
+                return UnboxResult.Succeed;
+            }
 
             if (inputObj is bool boolObj)
             {

src/Sage.Engine/Runtime/SageVariable.cs

@@ -38,6 +38,11 @@ public SageVariable(string name)
             this.Name = name;
         }
 
+        public static implicit operator bool(SageVariable other)
+        {
+            return SageValue.EqualTo(true, other);
+        }
+
         public override string? ToString()
         {
             return Value?.ToString();