feat(combine): support zero streams args to combine()

If combine() takes just the project function but no member streams, then it behaves like
xs.of(project()). This is useful when doing a spread over an array of member streams, and the array
happens to be empty.

src/core.ts

@@ -206,9 +206,12 @@ class CombineProducer<R> implements InternalProducer<R> {
 
   _start(out: InternalListener<R>): void {
     this.out = out;
-    const streams = this.streams;
-    for (let i = streams.length - 1; i >= 0; i--) {
-      streams[i]._add(new CombineListener(i, this));
+    const s = this.streams;
+    const L = s.length;
+    if (L == 0) this.zero(out); else {
+      for (let i = 0; i < L; i++) {
+        s[i]._add(new CombineListener(i, this));
+      }
     }
   }
 
@@ -224,6 +227,15 @@ class CombineProducer<R> implements InternalProducer<R> {
     this.vals = new Array(streams.length);
     this.ac = streams.length;
   }
+
+  zero(out: InternalListener<R>): void {
+    try {
+      out._n(this.project<R>());
+      out._c();
+    } catch (e) {
+      out._e(e);
+    }
+  }
 }
 
 export class FromArrayProducer<T> implements InternalProducer<T> {

tests/factory/combine.ts

@@ -73,6 +73,22 @@ describe('xs.combine', () => {
     });
   });
 
+  it('should handle a group of zero streams', (done) => {
+    const stream = xs.combine<string>(() => 'hi');
+    let expected = ['hi'];
+
+    stream.addListener({
+      next: (x) => {
+        assert.equal(x, expected.shift());
+      },
+      error: done,
+      complete: () => {
+        assert.equal(expected.length, 0);
+        done();
+      },
+    });
+  });
+
   it('should not break future listeners when CombineProducer tears down', (done) => {
     //     --0--1-2--|  innerA
     //     ---0---1--|  innerB
@@ -96,10 +112,10 @@ describe('xs.combine', () => {
       outer.shamefullySendNext(0);
     }, 100);
     setTimeout(() => {
-      innerA.shamefullySendNext(0)
+      innerA.shamefullySendNext(0);
     }, 150);
     setTimeout(() => {
-      innerB.shamefullySendNext(0)
+      innerB.shamefullySendNext(0);
     }, 175);
     setTimeout(() => {
       arrayInners.push(innerB);
@@ -109,7 +125,7 @@ describe('xs.combine', () => {
     setTimeout(() => {
       innerA.shamefullySendNext(2);
       outer.shamefullySendNext(2);
-      innerB.shamefullySendNext(1)
+      innerB.shamefullySendNext(1);
     }, 250);
     setTimeout(() => {
       innerA.shamefullySendComplete();