fix: don't wrap helpers that are not functions

- helpers should always be a function, but in #1639 one seems to
  be undefined. This was not a problem before 4.6 because helpers
  weren't wrapped then.
  Now, we must take care only to wrap helpers (when adding
  the "lookupProperty" function to the options), if they
  are really functions.

lib/handlebars/internal/wrapHelper.js

@@ -1,4 +1,9 @@
 export function wrapHelper(helper, transformOptionsFn) {
+  if (typeof helper !== 'function') {
+    // This should not happen, but apparently it does in https://github.com/wycats/handlebars.js/issues/1639
+    // We try to make the wrapper least-invasive by not wrapping it, if the helper is not a function.
+    return helper;
+  }
   let wrapper = function(/* dynamic arguments */) {
     const options = arguments[arguments.length - 1];
     arguments[arguments.length - 1] = transformOptionsFn(options);

spec/regressions.js

@@ -518,4 +518,13 @@ describe('Regressions', function() {
       sinon.restore();
     });
   });
+
+  describe("GH-1639: TypeError: Cannot read property 'apply' of undefined\" when handlebars version > 4.6.0 (undocumented, deprecated usage)", function() {
+    it('should treat undefined helpers like non-existing helpers', function() {
+      expectTemplate('{{foo}}')
+        .withHelper('foo', undefined)
+        .withInput({ foo: 'bar' })
+        .toCompileTo('bar');
+    });
+  });
 });