[fix] Sample should not produce anything before the source has produced any value

rx.lua

@@ -1229,7 +1229,9 @@ function Observable:sample(sampler)
     end
 
     local function onNext()
-      return observer:onNext(util.unpack(latest))
+      if #latest > 0 then
+        return observer:onNext(util.unpack(latest))
+      end
     end
 
     local function onError(message)

src/operators/sample.lua

@@ -16,7 +16,9 @@ function Observable:sample(sampler)
     end
 
     local function onNext()
-      return observer:onNext(util.unpack(latest))
+      if #latest > 0 then
+        return observer:onNext(util.unpack(latest))
+      end
     end
 
     local function onError(message)

tests/sample.lua

@@ -6,10 +6,7 @@ describe('sample', function()
   it('produces nil values if the sampler fires before the source does', function()
     local sampler = Rx.Observable.fromRange(3)
     local onNext, onError, onCompleted = observableSpy(Rx.Observable.empty():sample(sampler))
-    expect(#onNext).to.equal(3)
-    expect(next(onNext[1])).to_not.exist()
-    expect(next(onNext[2])).to_not.exist()
-    expect(next(onNext[3])).to_not.exist()
+    expect(#onNext).to.equal(0)
   end)
 
   it('produces the latest value produced by the source when the sampler fires', function()