Fix context-stack when calling block-helpers on null values

Fixes #1319

Original behaviour:
- When a block-helper was called on a null-context, an empty object was used
  as context instead. (#1093)
- The runtime verifies that whether the current context equals the
  last context and adds the current context to the stack, if it is not.
  This is done, so that inside a block-helper, the ".." path can be used
  to go back to the parent element.
- If the helper is called on a "null" element, the context was added, even
  though it shouldn't be, because the "null != {}"

Fix:
- The commit replaces "null" by the identifiable "container.nullContext"
  instead of "{}". "nullContext" is a sealed empty object.
- An additional check in the runtime verifies that the context is
  only added to the stack, if it is not the nullContext.

Backwards compatibility within 4.0.x-versions:
- This commit changes the compiler and compiled templates would not work
  with runtime-versions 4.0.0 - 4.0.6, because of the "nullContext"
  property. That's way, the compiled code reads
  "(container.nullContext || {})" so that the behavior will degrade
  gracefully with older runtime versions: Everything else will work
  fine, but GH-1319 will still be broken, if you use a newer compiler
  with a pre 4.0.7 runtime.

spec/regressions.js

@@ -277,4 +277,9 @@ describe('Regressions', function() {
 
     shouldCompileTo(string, { listOne: ['a'], listTwo: ['b']}, 'ab', '');
   });
+
+  it('GH-1319: "unless" breaks when "each" value equals "null"', function() {
+    var string = '{{#each list}}{{#unless ./prop}}parent={{../value}} {{/unless}}{{/each}}';
+    shouldCompileTo(string, { value: 'parent', list: [ null, 'a'] }, 'parent=parent parent=parent ', '');
+  });
 });