fix(plugin-helper): avoid some malformed attributes in attrsToString (#26)

* attrsToString: To avoid some malformed attributes

Error:
```
TypeError: Cannot convert undefined or null to object
    at Function.keys (<anonymous>)
    at attrsToString
```

This errors appears if no `attrs` setted in custom tag:
```
const BBcodePresetTemp = BbobPresetHTML5.extend((tags: any) => {
	tags.br = () => ({
		tag: 'br',
		// attrs: {}, // <-- Comment this line for error and add [br] to text
		content: null,
	});

	return tags;
});
```

packages/bbob-plugin-helper/test/index.test.js

@@ -79,4 +79,8 @@ describe('@bbob/plugin-helper', () => {
       disabled: true
     })).toBe(` tag="test" foo="bar" disabled`)
   })
+  
+  test('attrsToString undefined', () => {
+    expect(attrsToString(undefined)).toBe('')
+  })  
 });