Allow for fields to be optionally filled out in staff keyword search

Enhancement request to allow for the first search row to be optionally filled out in staff keyword search (maybe if other rows are present?).

With the addition of search templates in the staff search, the next logical step is for staff to create a template that tries to recreate the default Advanced Search interface (in 3.5.4 for example) where you had Keyword AND Title AND Author fields show up as a default, and you could optionally put a value in any of those fields, click Search and get results.

The staff Keyword search mechanism in 3.7.1 is designed for the first row to be required and the additional rows to be optional. If a staff member creates a search template with row 1 being Keyword, row 2 being Author and row 3 being Title, as a shortcut to have all their fields for their preferred search template, the search won't execute if you don't have row 1 filled out.

Thought this contradicts the design intention, this is pragmatic way that staff want to be able to set up their templates to have their search screen allow them to quickly enter the info they want to search without drop down menus or =/- buttons. Go to search, enter text in the relevant field of your default template and hit go.

(This is related to wanting to set a default search template) https://bugs.launchpad.net/evergreen/+bug/1946676