Fix getMany fails when called with an empty array of ids

[fzaninotto]

Closes #158

[coveralls]

Pull Request Test Coverage Report for Build 10682334781

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
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• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.3%) to 89.981%

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Totals
Change from base Build 9716866733:	0.3%
Covered Lines:	282
Relevant Lines:	293

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💛 - Coveralls