[BRLTTY] Fix python error report

[Samuel Thibault]

Hello,

I case brlapi didn't report a function name, we still need to set the
errfun property for the __str__ method below.

Samuel
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diff --git a/Bindings/Python/brlapi.pyx b/Bindings/Python/brlapi.pyx
index 5a20979..0e9672e 100644
--- a/Bindings/Python/brlapi.pyx
+++ b/Bindings/Python/brlapi.pyx
@@ -93,6 +93,8 @@ class OperationError(Exception):
 			self.gaierrno = c_brlapi.brlapi_error.gaierrno
 			if (c_brlapi.brlapi_error.errfun):
 				self.errfun = c_brlapi.brlapi_error.errfun
+			else:
+				self.errfun = b""
 
 	def __str__(self):
 		cdef c_brlapi.brlapi_error_t error