org.el (org-entry-get): Fix inheritance problem

org.el (org-entry-get): Fix inheritance problem

* org.el (org-entry-get): Only try to combine file properties
with local properties when the property drawer contains such
local property, not when the property drawer exists.

Before the fix, `org-entry-get' behavior was inconsistent,
returning `nil' when looking for a property in a subtree with
no property drawer, and returning the global properties in a
subtree with a property drawer, independantly of whether this
property drawer contained a reference to the property we are checking
against or not.

As a side-effect, inheritance was broken for the case that
Ilya reported in commit 475f2f53, because `org-entry-get' returned
a value (with the global properties of the file) too early.

Thanks to Ilya for raising this bug and to Achim for pointing
out that the previous fix was wrong.