Added a null check on the operators collection

[avgprog]

Resolves #264. Changes done:

• Added a null check on the operators Collection. If the Collection is null, OperatorInput.NO_OPERATOR_ASSIGNED will be returned by the getFirstOrDefaultOperator() function.

[codecov]

Codecov Report

Merging #313 (44bd86e) into dev (ba2860c) will increase coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##                dev     #313   +/-   ##
=========================================
  Coverage     77.98%   77.98%           
  Complexity     1951     1951           
=========================================
  Files           249      249           
  Lines          7935     7936    +1     
  Branches        749      749           
=========================================
+ Hits           6188     6189    +1     
  Misses         1321     1321           
  Partials        426      426           

Impacted Files	Coverage Δ	Complexity Δ
...edu/ie3/datamodel/io/source/csv/CsvDataSource.java	90.11% <0.00%> (+0.05%)	54.00% <0.00%> (+1.00%)

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