pySim-read.py: print meaningful message when SPN is not set

Change-Id: I1d98520f33a8564c7d69f50a0811204f138f9dca
diff --git a/pysim-testdata/sysmoISIM-SJA2.ok b/pysim-testdata/sysmoISIM-SJA2.ok
index 7346134..77c4878 100644
--- a/pysim-testdata/sysmoISIM-SJA2.ok
+++ b/pysim-testdata/sysmoISIM-SJA2.ok
@@ -3,7 +3,7 @@
 ICCID: 8988211900000000004
 IMSI: 001010000000102
 SMSP: ffffffffffffffffffffffffffffffffffffffffffffffffe1ffffffffffffffffffffffff0581005155f5ffffffffffff000000
-SPN: 
+SPN: Not available
 Display HPLMN: False
 Display OPLMN: False
 PLMNsel: fff11fffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff