[BEAM-9935] Counting empty split points as "any" in Go DataSource.

[youngoli]

Fix a small oversight in the split code. I check for nil to mean any splits are ok, but if the list is empty instead of nil it should have the same behavior.

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Post-Commit Tests Status (on master branch)

Lang	SDK	Dataflow	Flink	Samza	Spark
Go		---		---
Java
Python				---
XLang	---	---		---	---

Pre-Commit Tests Status (on master branch)

---	Java	Python	Go	Website
Non-portable
Portable	---		---	---

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[youngoli]

R: @lostluck CC: @lukecwik

[lostluck]

This is something I should have caught on initial review too.

Just check the length, it's valid and cleaner. The times are rare when one wants to distinguish between a nil slice and a zero length one.

[youngoli]

I forgot that you could call len on nil slices, that's much better. Done.

[lostluck]

LGTM thanks!