templates: show image file table only if clients have a cache

Hide form table when the client's cache is empty and inform the
user about the selected clients not having images in cache.

ogcp/templates/actions/cache.html

@@ -20,6 +20,7 @@
 
 <p>{{ _('Select the images to be deleted:') }}</p>
 
+{% if form.images|length > 1 %}
 <form class="form-inline" method="POST" id="cacheForm">
     <table class="table table-hover">
         <thead class="thead-light">
@@ -48,6 +49,11 @@
         {{ _('Delete') }}
     </button>
 </form>
+{% else %}
+<div class="card text-center">
+    <b>{{ _('Cache is currently empty in the selected client(s)') }}</b>
+</div>
+{% endif %}
 
 <br>