Check for None request before sending them to crawl

make_request_from_url on InitSpider returns None for the requests scheduled for postinit.

When using InitSpider and start_urls make_request_from_url  from InitSpider returns None for each url after the first (scheduling them instead of returning right away), added check on engine.py/_next_request

Tried with this sample spider:
```python
from scrapy.http import Request
from scrapy.contrib.spiders.init import InitSpider


class DmozSpider(InitSpider):
    name = "dmoz"
    allowed_domains = ["dmoz.org"]
    start_urls = [
        "http://www.dmoz.org/Computers/Programming/Languages/Python/Books/",
        "http://www.dmoz.org/Computers/Programming/Languages/Python/Resources/"
    ]

    def init_request(self):
        return Request("http://www.dmoz.org", callback=self.initialized)

    def parse(self, response):
        filename = response.url.split("/")[-2]
        open(filename, 'wb').write(response.body)
```