Don't fail when there is a reference without a formula at a given instant.

src/lib/components/variables/VariableView.svelte

@@ -174,7 +174,11 @@
       {#each buildInstantFormulaAndReferencesArray(variable) as { formula, instant, references }}
         <div class="mt-10">
           <h2 class="mr-1 mb-1 font-serif font-bold text-xl pt-7 pb-1">
-            {#if instant === "0001-01-01"}
+            {#if formula === undefined}
+              {#if instant !== "0001-01-01"}
+                Le <i>{instant}</i>&nbsp;:
+              {/if}
+            {:else if instant === "0001-01-01"}
               {#if Object.keys(variable.formulas).length > 1}
                 Formule <i>initiale</i> de calcul OpenFisca&nbsp;:
               {:else}
@@ -185,11 +189,13 @@
             {/if}
           </h2>
 
+          {#if formula !== undefined}
             {#if formula === null}
               <i>Aucune formule à partir de cette date</i>
             {:else}
               <FormulaView {formula} bind:situation {year} />
             {/if}
+          {/if}
 
           {#if references.length > 0}
             <section>