Breaks if no package.json is present

If there is no `package.json` available then the path is `null` and you can't stop it from breaking.

Even if you manually provide the `projectName` in options it will still break. This adds a few simple checks to prevent it from blowing up when you do provide the name.

I've also added an appropriate test, which properly fails when run without the changes to `index.js` and correctly passes once added.

Closes #6

index.js

@@ -15,7 +15,11 @@ const parentDir = path.dirname(module.parent.filename);
 class Conf {
 	constructor(opts) {
 		const pkgPath = pkgUp.sync(parentDir);
-		opts = Object.assign({projectName: require(pkgPath).name}, opts);
+
+		opts = Object.assign({
+			// if the package.json was not found, avoid breaking with `require(null)`
+			projectName: pkgPath && require(pkgPath).name
+		}, opts);
 
 		if (!opts.projectName && !opts.cwd) {
 			throw new Error('Project name could not be inferred. Please specify the `projectName` option.');

test.js

@@ -2,6 +2,7 @@ import path from 'path';
 import {serial as test} from 'ava';
 import tempfile from 'tempfile';
 import del from 'del';
+import pkgUp from 'pkg-up';
 import Conf from './';
 
 const fixture = '🦄';
@@ -161,3 +162,15 @@ test('`cwd` option overrides `projectName` option', t => {
 	t.is(conf.get('foo'), fixture);
 	del.sync(conf.path, {force: true});
 });
+
+test('safely handle missing package.json', t => {
+	const pkgUpSyncOrig = pkgUp.sync;
+	pkgUp.sync = () => null;
+
+	let conf;
+	t.notThrows(() => {
+		conf = new Conf({projectName: 'conf-fixture-project-name'});
+	});
+	del.sync(conf.path, {force: true});
+	pkgUp.sync = pkgUpSyncOrig;
+});