added missing exists

as a respective reference to ObjectExists
Merge pull request #306 from esserj/patch-1

added missing exists
Ensure that tryConvertArrayToObject returns an object.

If an ID value is passed which is not found, previously no object would
be returned, eventually causing the hydration to return a null instead
of an object.

One situation in which this is likely to occur is when an external ID is
used as a primary key; in this case, the generator is NONE, but because
the id value is provided externally it is passed into the inflation.
For new entities, this will not be found, so null is returned and
hydration fails.

I'm not completely sure of the implications of this; tests pass, but are
there some situations out there where returning an object here instead
of null will break things?