Fix nil pointer when DS Agent disabled

[ahmed-mez]

What does this PR do?

Do not panic when node agent isn't enabled

Motivation

Panics are bad

[codecov-commenter]

Codecov Report

Merging #110 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #110   +/-   ##
=======================================
  Coverage   57.44%   57.44%           
=======================================
  Files          31       31           
  Lines        4199     4199           
=======================================
  Hits         2412     2412           
  Misses       1606     1606           
  Partials      181      181           

Flag	Coverage Δ
#unittests	57.44% <100.00%> (ø)

Impacted Files	Coverage Δ
pkg/controller/datadogagent/systemprobe.go	100.00% <100.00%> (ø)

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