custom wp_query where meta_key is variable or not needed - wordpress

Question

To make a custom post type filterable and sortable, i created a custom query that works based on variables in url, accessed by $_GET.
One specific situation breaks it. This is an (un)specified metakey. In the case of sort, the metakey is needed when sorting on a custom field. In the other cases, the metakey can be ignored. However, when i set the variable on empty, the query doesnt produce any posts. How to deal with an empty meta_key?
So far i tried to set the variable as empty ($variable ='');
I set the variable as null; I have used unset.
if (isset($_GET["key"]) || isset($_GET["orderby"])){
if (isset($_GET["key"])) {$key = $_GET["key"];}
else {$key = '';}
if (isset($_GET["value"])) {$value = $_GET["value"]; echo $value;}
else {$value = '';}
if (isset($_GET["orderby"])) {$orderby = $_GET["orderby"];}
if ($orderby='meta_value'){$meta_key='averagerating';}
else {$orderby='';$meta_key='';}
if (isset($_GET["order"])) {$order = $_GET["order"];}
else {$order='';}
$cat = get_queried_object();
if (!empty($cat) && !is_post_type_archive('bedrijf')){
$category = $cat->name;
}
if (is_post_type_archive('bedrijf')){
$category = '';
$terms = get_terms( 'bedrijfs-category' );
// convert array of term objects to array of term IDs
$category = wp_list_pluck( $terms, 'name' );
}
global $post;
$the_query ='';
$args = array(
'post_type' => 'bedrijf',
'posts_per_page' => 10,
'meta_key' => ''.$meta_key.'',
'orderby' => ''.$orderby.'',
'order' => ''.$order.'',
'tax_query' => array(
array(
'taxonomy' => 'bedrijfs-category',
'field' => 'name',
'terms' => $category
)
),
'meta_query' => array(
array(
'key'=> ''.$key.'',
'value' => ''.$value.'',
'compare' => 'LIKE'
)
)
);
$the_query = new WP_Query($args); }
WHat i would like is a solution for inserting an empty variable in the meta_key => ''.$meta_key.'' so the loop skips the meta_key part.

Answer 1

define meta_query or tax_query out of $args
and (example):
if($_GET['some_name'] !== ''){
$meta_query = array(
array(
'taxonomy' => 'bedrijfs-category',
'field' => 'name',
'terms' => $category
)
);
//define args with some_one para exists
$args = array(
'post_type' => 'bedrijf',
'posts_per_page' => 10,
'meta_key' => ''.$meta_key.'',
'orderby' => ''.$orderby.'',
'order' => ''.$order.'',
'meta_query' => $meta_query
);
}else{
//define args with some_one para not exists
$args = array(
'post_type' => 'bedrijf',
'posts_per_page' => 10,
'meta_key' => ''.$meta_key.'',
'orderby' => ''.$orderby.'',
'order' => ''.$order.''
);
}
tax_query is like this