ParseAndRenderString() panics for certain arguments

[magiusdarrigo]

Checklist

• I have searched the issue list
• I have tested my example against Shopify Liquid. (This isn't necessary if the actual behavior is a panic, or an error for which IsTemplateError returns false.)

Expected Behavior

Gracefully returns an error

Actual Behavior

Panics

Detailed Description

I have the line of code:

result, err := lqEngine.ParseAndRenderString(template, bindings)
if template equals the string: {% if %} the line panics.

Possible Solution

PR

[magiusdarrigo]

Pull request with patch made - #91

cc: @danog