fix(pipe): Special handling for 0-arg case. (#4936)

* fix(pipe): Special handling for 0 args.

* test(pipe): Fix test label for 0-arg.

* fix(pipe): Remove check for missing args array.

The check is never reached in internal code, and is not exported for
external code. The behavior is not tested for, and arguably wrong.

* fix(pipe): no argument case run-time check

* refactor: revert checking for lack of array argument

Co-authored-by: Ben Lesh <ben@benlesh.com>

spec/util/pipe-spec.ts

@@ -23,7 +23,7 @@ describe('pipe', () => {
     expect(c).to.equal(a);
   });
 
-  it('should return a noop if not passed a function', () => {
+  it('should return the identity if not passed any functions', () => {
     const c = pipe();
 
     expect(c('whatever')).to.equal('whatever');

src/internal/util/pipe.ts

@@ -1,4 +1,5 @@
 import { noop } from './noop';
+import { identity } from './identity';
 import { UnaryFunction } from '../types';
 
 /* tslint:disable:max-line-length */
@@ -21,8 +22,8 @@ export function pipe(...fns: Array<UnaryFunction<any, any>>): UnaryFunction<any,
 
 /** @internal */
 export function pipeFromArray<T, R>(fns: Array<UnaryFunction<T, R>>): UnaryFunction<T, R> {
-  if (!fns) {
-    return noop as UnaryFunction<any, any>;
+  if (fns.length === 0) {
+    return identity as UnaryFunction<any, any>;
   }
 
   if (fns.length === 1) {