Search (comb) throws exception if search term starts with plus (+) or minus (-) sign

[pxmch]

Describe the bug If a search term starts with a plus (+) or a minus (-) sign, comb throws the following exception: ErrorException in Comb.php line 714: Array to string conversion

To Reproduce

1. Set up a search form with this code:

   <form method="get" action="/varia/search" id="search-form">
   <input id="site-search" type="text" value="" class="search-query" name="q">
   <input type="submit" value="Suche" name="Suche" class="btn-search">
   </form>

2. Enter one of the following search terms: +term1 or -term1 or term1 +term2 or term1 -term2
3. Submit the form.

Expected behavior The search should return some or no results but not throw an exception

Screenshots

Environment details (please complete the following information):

• Statamic Version 2.11.9
• Fresh Install or Upgrade: fresh install
• OS: macOS 10.13.6
• Browser: any browser
• Web Server: Apache
• PHP Version: 7.2
• Addons installed: none