Fix crash in vortex subscriber when there is no filter

[dstiliadis]

Fixes the crash in vortex manipulator where it was assuming that the filter is always not nil.

[ghost]

/build - automatically fired by gogo with following PRs and commit SHAs v1.0.0

[
  {
    "project": "",
    "component": "manipulate",
    "pr-id": "97",
    "commit-sha": "f1a84800e971e16b454bef6650fc34a90fb37140"
  }
]

[codecov]

Codecov Report

Merging #97 into master will decrease coverage by 0.05%.
The diff coverage is 25%.

@@            Coverage Diff             @@
##           master      #97      +/-   ##
==========================================
- Coverage   76.81%   76.76%   -0.06%     
==========================================
  Files          35       35              
  Lines        2566     2569       +3     
==========================================
+ Hits         1971     1972       +1     
- Misses        517      519       +2     
  Partials       78       78

Impacted Files	Coverage Δ
manipvortex/manipulator.go	69.37% <25%> (-0.29%)	⬇️

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