Fixed missing middlename action

ref #79

src/components/donations/AddDonationModal.svelte

@@ -209,7 +209,7 @@
                     {#each donors as d}
                       <option value={d.id}>
                         {d.firstname}
-                        {d.middlename}
+                        {d.middlename || ''}
                         {d.lastname}
                       </option>
                     {/each}
@@ -226,7 +226,7 @@
                       {#each runners as r}
                         <option value={r.id}>
                           {r.firstname}
-                          {r.middlename}
+                          {r.middlename || ''}
                           {r.lastname}
                         </option>
                       {/each}