BigQuery: use _get_sub_prop helper so missing load stats don't raise.

[hrfuller]

No description provided.

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CLAs look good, thanks!

[tseaver]

@dpebot merge when green, please.

[dpebot]

Okay! I'll merge when all statuses are green and all reviewers approve.

[tseaver]

@theacodes I have no idea why @dpebot drops the ball on this one.

[tseaver]

@hrfuller Thanks for the patch!