Clean up 0-length handling in gdbpy_create_lazy_string_object

gdbpy_create_lazy_string_object will throw an exception if you pass it
a NULL pointer without also setting length=0 -- the default,
length==-1, will fail.  This seems bizarre.  Furthermore, it doesn't
make sense to do this check for array types, as an array can have a
zero length.  This patch cleans up the check and makes it specific to
TYPE_CODE_PTR.