Check that url_rule is not None before dereferencing property

[jtbeach]

Fixes #777

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[jtbeach]

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[olance]

@jtbeach you should improve the tests I had provided for the HTTP_ROUTE attribute, to add one that specifically tests a non-existing route.

Happy to help if needed!

[jtbeach]

@olance @reyang I think this is now ready for review

[reyang]

LGTM

[reyang]

Thanks @jtbeach and @olance!

[jtbeach]

@reyang Any idea when a release will be made with this fix?

[lzchen]

@jtbeach
https://github.com/census-instrumentation/opencensus-python/releases/tag/v0.7.5