Feature/issue 683 size 0 input

#### Summary:

Generalize R dump format parser to allow zero-size array and vector inputs, solving issue #683.

#### Intended Effect:

R dump format parser now accepts zero-size array and vector inputs. 

#### How to Verify:

Unit test in `unit/io/dump_test.hpp`.  Can also try to compile and run Bernoulli model to test zero-length integers in practice, or a simple normal with a prior on mu and no data.

#### Side Effects:

No.

#### Documentation:

The documentation for the dump format is in CmdStan's guide never ruled out zero-size inputs, which is why this is really a bug fix rather than a new feature.

#### Reviewer Suggestions:

Anyone.