fix: Accept undefined pluginConfig

[pvdlg]

Similar to semantic-release/commit-analyzer#12

Now that the plugin is the default one, a configuration in the release.generateNotes is not mandatory, therefore the plugin has to handle the case where pluginConfig is undefined.

[codecov]

Codecov Report

Merging #16 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #16   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           2      2           
  Lines          35     35           
=====================================
  Hits           35     35

Impacted Files	Coverage Δ
lib/index.js	100% <100%> (ø)	⬆️

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[gr2m]

I wonder if this is something we should fix in semantic-release core, so that pluginConfig is always passed (defaulting to {})?

[pvdlg]

I wonder if this is something we should fix in semantic-release core, so that pluginConfig is always passed (defaulting to {})?

We could but that concerns only the default plugins.
For 3rd parties plugins I think there is always a pluginOption as it's where the custom plugin is defined in the release key of package.json