How to do a particular wp_query taxonomy search

Question

I have added a custom taxonomy called experience to the page post-type, and the taxonomy has terms "vfds" and "plcs"

I am trying to create a search where a user can select one or more experience taxonomy terms from a select list, press the submit button, and see the results of a wp_query.

I need the query result to show pages with

1) the experience taxonomy has no (zero) terms selected

2) one or more terms selected

I have an html select list with an array experience[] to capture multiple selections:

<select name="experience[]" multiple="multiple">
<option value="vfds">VFDs</option>
<option value="plcs">PLCs</option>
</select> 

Then I feed this array into a wp_query:

    <?php

        $_GET['submit'];
        $experience = $_GET['experience'];
        $args = array
        (   
                'post_type' => 'page',      
                'tax_query' => array(
                    array(
                        'taxonomy' => 'experience1',
                        'field' => 'slug',
                        'terms' => $experience
                    )
                 )
        );
    $query = new WP_Query( $args );

The query works fine in the sense that it will show pages that have one or more terms selected under the experience category.

The problem I am having is this:

IF a page has empty (no) values for the experience category, then it is not shown in the result list.

Is there a way where wp_query will show pages with

1. no terms selected
2. one or more terms selected

Thank you

---

EDIT #1

Please let me add that in general I am trying to move away from using Drupal with Views. I am trying to replicate Views capability of being able to search using select fields with multiple values.

I have tried to use wp_query with a custom select field with multiple values, and failed miserably so far.

Hence I switched to using categories with multiple terms and am close to replicating a Views type search result.

If anyone knows any other way of doing what I would like to do in WP, I would very much appreciate it.

Thanks again!

---

EDIT #2

I realize maybe my question does not make full sense. Let me try and explain further. The reason I need to search result to show pages with ZERO or more terms instead of ONE or more terms, is that I would like to combine the taxonomy search with a another field search, eg City. If a user fills in only a value for city, and no value(s) for experience terms, the search results will be zero because he did not fill in terms. Instead I need to show all pages with the city that was selected, whether or not any experience terms are selected

Answer 1

Seems like the easiest thing to do is to apply the tax query only if at least one "experience" term has been specified. Something like:

// Get passed vars
$experience = $_GET['experience'];

// Start building the args array
$args = array(   
    'post_type' => 'page',      
);

// If any experience items were passed
if( is_array( $experience ) && count( $experience ) > 0 ) {

    // Get an array of possible "experience" terms as a whitelist to check against.
    $arr_term_details = get_terms( 'experience1', array( 'hide_empty' => 0 ) );
    $arr_terms = array();
    foreach($arr_term_details as $this_term) {
        array_push($arr_terms, $this_term->slug);
    }

    // If ALL of the experience terms exist in the whitelist
    if( count(array_intersect( $experience, $arr_terms )) == count($experience) ) {

        // Add the tax query
        $args['tax_query'] = array(
            array(
                'taxonomy' => 'experience1',
                'field' => 'slug',
                'terms' => $experience
            )
        );

    }

}

// Run the query
$query = new WP_Query( $args );

The if condition may be wrong--I forget how forms pass values for multi-selects.

UPDATE: added a whitelist-style check for the $_GET data, per Chip Bennett's suggestion.