filesystem: pass total_len to construct of when encoding file contents

In our construct models we frequently use a context parameter "total_len",
we also pass this parameter to construct when we decode files, but we
do not pass it when we generate files. This is a problem, because when
total_len is used in the construct model, this parameter must be known
also when decoding the file.

Let's make sure that the total_len is properly determined and and passed
to construct (via pyosmocom).

Related: OS#5714
Change-Id: I1b7a51594fbc5d9fe01132c39354a2fa88d53f9b

tests/pySim-shell_test/file_content/test_record_uicc.ok

@@ -12,7 +12,7 @@
 010 0123456789abcdef0123456789abcdef0123456789abcdef0123456789
 [
     {
-        "alpha_id": null,
+        "alpha_id": "",
         "len_of_bcd": 7,
         "ton_npi": {
             "ext": true,
@@ -60,7 +60,7 @@
         "ext4_record_id": 255
     },
     {
-        "alpha_id": null,
+        "alpha_id": "",
         "len_of_bcd": 7,
         "ton_npi": {
             "ext": true,
@@ -120,7 +120,7 @@
         "ext4_record_id": 255
     },
     {
-        "alpha_id": null,
+        "alpha_id": "",
         "len_of_bcd": 7,
         "ton_npi": {
             "ext": true,
@@ -133,7 +133,7 @@
     }
 ]
 {
-    "alpha_id": null,
+    "alpha_id": "",
     "len_of_bcd": 7,
     "ton_npi": {
         "ext": true,