build: ensure we do not set undefined releaseType

---
type: pre_commit_static_analysis_report
description: Results of running static analysis checks when committing changes.
report:
  - task: lint_filenames
    status: passed
  - task: lint_editorconfig
    status: passed
  - task: lint_markdown
    status: na
  - task: lint_package_json
    status: na
  - task: lint_repl_help
    status: na
  - task: lint_javascript_src
    status: passed
  - task: lint_javascript_cli
    status: na
  - task: lint_javascript_examples
    status: na
  - task: lint_javascript_tests
    status: na
  - task: lint_javascript_benchmarks
    status: na
  - task: lint_python
    status: na
  - task: lint_r
    status: na
  - task: lint_c_src
    status: na
  - task: lint_c_examples
    status: na
  - task: lint_c_benchmarks
    status: na
  - task: lint_c_tests_fixtures
    status: na
  - task: lint_shell
    status: na
  - task: lint_typescript_declarations
    status: na
  - task: lint_typescript_tests
    status: na
  - task: lint_license_headers
    status: passed
---

Author: Planeshifter

lib/node_modules/@stdlib/_tools/scripts/publish_packages.js

@@ -685,9 +685,15 @@ function publish( pkg, clbk ) {
 		return invokeCallback( null, 'skipped' );
 	}
 
-	changelog = generateChangelog( '@stdlib/'+pkg, {
-		'releaseType': flags[ 'release-type' ]
-	});
+	if ( flags[ 'release-type' ] ) {
+		// Case: release type specified, so we generate a changelog for the specified release type...
+		changelog = generateChangelog( '@stdlib/'+pkg, {
+			'releaseType': flags[ 'release-type' ]
+		});
+	} else {
+		// Case: no release type specified, so we generate a changelog for a non-release...
+		changelog = generateChangelog( '@stdlib/'+pkg );
+	}
 	releaseType = changelog.releaseType;
 
 	mainJSON = readJSON( join( mainDir, 'package.json' ) );