Fix throw when toggling Tor with unopened cryptonote family wallets

[sneurlax]

When you toggle Tor with an uninitialized cryptonote wallet, it throws. This isn't fatal. The fix uses the same guard pattern other functions use.

This fix is admittedly optional: it's not a critical fix but rather a cosmetic/log-cleaning fix.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 131 lines in your changes missing coverage. Please review.
✅ Project coverage is 4.93%. Comparing base (cf7251b) to head (adbeade).
⚠️ Report is 1 commits behind head on staging.

Files with missing lines	Patch %	Lines
...wallets/wallet/intermediate/lib_monero_wallet.dart	0.00%	45 Missing ⚠️
...allets/wallet/intermediate/lib_wownero_wallet.dart	0.00%	44 Missing ⚠️
...allets/wallet/intermediate/lib_salvium_wallet.dart	0.00%	42 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           staging    #1275      +/-   ##
===========================================
- Coverage     4.93%    4.93%   -0.01%     
===========================================
  Files         1064     1064              
  Lines       107980   108022      +42     
===========================================
+ Hits          5328     5330       +2     
- Misses      102652   102692      +40     

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[sneurlax]

Comments:
"""
does this null check https://github.com/cypherstack/stack_wallet/pull/1275/changes hide a problem elsewhere?
I'm looking at 1275 again. If the null check hides a problem elsewhere and that problem was fixed, why is the null check still there?
any chance introducing another mutex (_updateNodeMutex) could cause a deadlock?
Ok so I am generally in favor of writing less code and doing the same check in a single place instead of spreading it all over the place but I still think the null check is going a bit too far in catching every single call.

For example in https://www.github.com/cypherstack/stack_wallet/blob/211971b5d7c6cac19173071026388dbaf756e8ab/lib/pages/settings_views/global_settings_view/manage_nodes_views/add_edit_node_view.dart#L306-L339
The check should probably be handled here as refresh is called on the same wallets. Maybe I'm overthinking it though?
"""