fix: Accept undefined values for the getLastRelease and generateNotes plugins

Adapt tests and default value to the plugin definitions (`definitions.js`)

lib/get-commits.js

@@ -18,6 +18,7 @@ const getVersionHead = require('./get-version-head');
  * @typedef {Object} LastRelease
  * @property {string} version The version number of the last release.
  * @property {string} [gitHead] The commit sha used to make the last release.
+ * @property {string} [gitTag] The tag used to make the last release.
  */
 
 /**
@@ -45,7 +46,7 @@ const getVersionHead = require('./get-version-head');
  * @throws {SemanticReleaseError} with code `ENOTINHISTORY` if `lastRelease.gitHead` or the commit sha derived from `config.lastRelease.version` is not in the direct history of `branch`.
  * @throws {SemanticReleaseError} with code `ENOGITHEAD` if `lastRelease.gitHead` is undefined and no commit sha can be found for the `config.lastRelease.version`.
  */
-module.exports = async ({version, gitHead}, branch, logger) => {
+module.exports = async ({version, gitHead} = {}, branch, logger) => {
   let gitTag;
   if (gitHead || version) {
     try {