fix: npe for destination table

[pmakani]

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

Fixes #697

[codecov]

Codecov Report

Merging #699 into master will decrease coverage by 0.00%.
The diff coverage is 75.00%.

@@ Coverage Diff @@ ## master #699 +/- ## ============================================ - Coverage 81.07% 81.06% -0.01%  Complexity 1236 1236 ============================================ Files 78 78 Lines 6329 6332 +3 Branches 702 704 +2 ============================================ + Hits 5131 5133 +2  Misses 833 833 - Partials 365 366 +1 

Impacted Files	Coverage Δ	Complexity Δ
...y/src/main/java/com/google/cloud/bigquery/Job.java	85.91% <75.00%> (-0.42%)	32.00 <0.00> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 225c8c1...638370f. Read the comment docs.