Change default gender option to be blank.

lms/templates/signup_modal.html

@@ -46,6 +46,7 @@
             <label>Gender</label>
             <div class="input-wrapper">
               <select name="gender">
+                  <option value="">--</option>
                   %for code, gender in UserProfile.GENDER_CHOICES:
                       <option value="${code}">${gender}</option>
                   %endfor