shared-state check for babeld file existence before reading it

[ilario]

When installing shared-state without Babeld, the following error appears every time shared-state is used:

# shared-state sync dnsmasq-hosts
grep: /var/etc/babeld.conf: No such file or directory

I tested the proposed change and executes the code only if that file is present.

Tell me if you prefer I open also an issue on this.

[codecov-commenter]

Codecov Report

Merging #983 (94f6d6c) into master (26ab694) will not change coverage.
The diff coverage is n/a.

❗ Current head 94f6d6c differs from pull request most recent head 3c64479. Consider uploading reports for the commit 3c64479 to get more accurate results

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@@           Coverage Diff           @@
##           master     #983   +/-   ##
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  Coverage   77.84%   77.84%           
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  Files          52       52           
  Lines        4388     4388           
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  Hits         3416     3416           
  Misses        972      972           

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[ilario]

Tested this on a router running OpenWrt 22.03