fix: don't try to shut cassandra client down if it wasn't initialized

[rauno56]

Which problem is this PR solving?

Tests fail if Cassandra tests which require an external service to be running are not run.

Short description of the changes

Add conditional shut down to avoid running methods on uninitialized var.

It seems like codecov check fails because Cassandra tests covered less than the previous average, thus bringing the avg down.

[codecov]

Codecov Report

Merging #966 (2594a05) into main (cfb0b7a) will decrease coverage by 2.22%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #966      +/-   ##
==========================================
- Coverage   95.91%   93.69%   -2.23%     
==========================================
  Files          13       15       +2     
  Lines         856     1062     +206     
  Branches      178      209      +31     
==========================================
+ Hits          821      995     +174     
- Misses         35       67      +32     

Impacted Files	Coverage Δ
...y-instrumentation-cassandra/src/instrumentation.ts	80.37% <0.00%> (ø)
...tapackages/auto-instrumentations-node/src/utils.ts	97.91% <0.00%> (ø)