app_persistency: file open fix

https://git.debu.gs/gitweb/?p=bmcd;a=commitdiff;h=38c5af03064d6c1ed1e1a6d5a41aa6439c93b6e4
code, git.debu.gs | 2024-02-14 13:11:48 +0000 GMT | No.1458 
app_persistency: file open fix

An error was logged when there is no persistency file available on the
system. Even though the bmcd recover from this quickly, a fix was
made so that no attempt is made to open a non existing file.
  • [DH] src/persistency/app_persistency.rs