Fix exception when an object has a nil description

An exception is thrown when an object parameter of an invocation has a nil description as the result of [object description] is directly passed to appendString: in the invocationDescription method.  This pull request fixes the issue by returning an empty string instead of nil.
Added comment
Update NSInvocation+OCMAdditions.m
Update NSInvocation+OCMAdditions.m
Merge pull request #76 from nrbrook/patch-1

Fix exception when an object has a nil description