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.

lib/handlebars/runtime.js

@@ -124,6 +124,8 @@ export function template(templateSpec, env) {
 
       return obj;
     },
+    // An empty object to use as replacement for null-contexts
+    nullContext: Object.seal({}),
 
     noop: env.VM.noop,
     compilerInfo: templateSpec.compiler
@@ -187,7 +189,7 @@ export function template(templateSpec, env) {
 export function wrapProgram(container, i, fn, data, declaredBlockParams, blockParams, depths) {
   function prog(context, options = {}) {
     let currentDepths = depths;
-    if (depths && context != depths[0]) {
+    if (depths && context != depths[0] && !(context === container.nullContext && depths[0] === null)) {
       currentDepths = [context].concat(depths);
     }