Fixed missing middlename action

ref #79

src/components/donations/DonationDetail.svelte

@@ -207,7 +207,7 @@
         {#each current_donors as d}
           <option value={d.id}>
             {d.firstname}
-            {d.middlename}
+            {d.middlename || ''}
             {d.lastname}
           </option>
         {/each}
@@ -224,7 +224,7 @@
           {#each current_runners as r}
             <option value={r.id}>
               {r.firstname}
-              {r.middlename}
+              {r.middlename || ''}
               {r.lastname}
             </option>
           {/each}