Assume first record number if caller specifies none

This fixes a regression introduced in Change-Id
I02d6942016dd0631b21d1fd301711c13cb27962b which added support for
different encoding/decoding of records by their record number.

Change-Id: I0c5fd21a96d2344bfd9551f31030eba0769636bf
diff --git a/pySim/filesystem.py b/pySim/filesystem.py
index 6dd1db7..ce1882b 100644
--- a/pySim/filesystem.py
+++ b/pySim/filesystem.py
@@ -945,7 +945,7 @@
         self._construct = None
         self._tlv = None
 
-    def decode_record_hex(self, raw_hex_data: str, record_nr: int) -> dict:
+    def decode_record_hex(self, raw_hex_data: str, record_nr: int = 1) -> dict:
         """Decode raw (hex string) data into abstract representation.
 
         A derived class would typically provide a _decode_record_bin() or _decode_record_hex()