Fix NPE when line number is not present in model

[uhafner]

API methods that return a collection should never return null.

Fixes https://github.com/jenkinsci/code-coverage-api-plugin/issues/426

[codecov]

Codecov Report

Merging #428 (2166351) into master (58ee93a) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master     #428   +/-   ##
=========================================
  Coverage     72.18%   72.18%           
  Complexity      992      992           
=========================================
  Files            86       86           
  Lines          3700     3700           
  Branches        433      433           
=========================================
  Hits           2671     2671           
  Misses          883      883           
  Partials        146      146           

Impacted Files	Coverage Δ
...nkins/plugins/coverage/targets/CoverageResult.java	33.00% <100.00%> (ø)

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