chore: add input validation for formatName

Refs: #2462

Author: simschla

lib/src/main/java/com/diffplug/spotless/npm/EslintFormatterStep.java

@@ -69,6 +69,7 @@ public static Map<String, String> defaultDevDependenciesWithEslint(String versio
 	}
 
 	public static FormatterStep create(String formatName, Map<String, String> devDependencies, Provisioner provisioner, File projectDir, File buildDir, File cacheDir, NpmPathResolver npmPathResolver, EslintConfig eslintConfig) {
+		requireNonNull(formatName);
 		requireNonNull(devDependencies);
 		requireNonNull(provisioner);
 		requireNonNull(projectDir);

lib/src/main/java/com/diffplug/spotless/npm/PrettierFormatterStep.java

@@ -52,6 +52,7 @@ public static final Map<String, String> defaultDevDependenciesWithPrettier(Strin
 	}
 
 	public static FormatterStep create(String formatName, Map<String, String> devDependencies, Provisioner provisioner, File projectDir, File buildDir, File cacheDir, NpmPathResolver npmPathResolver, PrettierConfig prettierConfig) {
+		requireNonNull(formatName);
 		requireNonNull(devDependencies);
 		requireNonNull(provisioner);
 		requireNonNull(buildDir);