Detect undefined variables (#844)

* #64: Ensure variables completeness with `StrictVariables` option

* #64: Replace strict variable buffering with UndefinedValue sentinel

* Refactor to use Nil

* Update documentation

* Update variable name

* Update Fluid/Values/ObjectValueBase.cs

Co-authored-by: Copilot <[email protected]>

* Update Fluid.Tests/StrictVariableTests.cs

Co-authored-by: Copilot <[email protected]>

* Re-introduce UndefinedValue

* Fix build

---------

Co-authored-by: compyle-bot[bot] <238113443+compyle-bot[bot]@users.noreply.github.com>
Co-authored-by: Sebastien Ros <[email protected]>
Co-authored-by: Copilot <[email protected]>

Author: 4 people

Fluid.Tests/FromStatementTests.cs

@@ -62,7 +62,7 @@ Hello world!
         await fromStatement.WriteToAsync(sw, HtmlEncoder.Default, context);
 
         Assert.IsType<FunctionValue>(context.GetValue("hello_world"));
-        Assert.IsType<NilValue>(context.GetValue("hello"));
+        Assert.IsType<UndefinedValue>(context.GetValue("hello"));
     }
 
     [Fact]

Fluid.Tests/MiscFiltersTests.cs

@@ -720,7 +720,7 @@ public async Task JsonShouldWriteNullIfDictionaryNotReturnFluidIndexable()
             options.MemberAccessStrategy.Register(model.GetType());
             var input = FluidValue.Create(model, options);
             var result = await MiscFilters.Json(input, new FilterArguments(), new TemplateContext(options));
-            Assert.Equal("{\"Id\":1,\"WithoutIndexable\":{\"Type\":5,\"Value\":{}},\"Bool\":true}", result.ToStringValue());
+            Assert.Equal("{\"Id\":1,\"WithoutIndexable\":{\"Type\":6,\"Value\":{}},\"Bool\":true}", result.ToStringValue());
         }
 
         [Fact]
@@ -910,7 +910,7 @@ public async Task HmacSha1(string key, string value, string expected)
         {
             // Arrange
             FluidValue input = value is null
-                ? NilValue.Empty
+                ? EmptyValue.Instance
                 : new StringValue(value);
             var arguments = new FilterArguments(FluidValue.Create(key, TemplateOptions.Default));
             var context = new TemplateContext();
@@ -932,7 +932,7 @@ public async Task HmacSha256(string key, string value, string expected)
         {
             // Arrange
             FluidValue input = value is null
-                ? NilValue.Empty
+                ? EmptyValue.Instance
                 : new StringValue(value);
             var arguments = new FilterArguments(FluidValue.Create(key, TemplateOptions.Default));
             var context = new TemplateContext();