Update picker to use null as default placeholder value

packages/standard-components/src/forms/Form.svelte

@@ -35,7 +35,7 @@
 
   // Form API contains functions to control the form
   const formApi = {
-    registerField: (field, defaultValue = undefined, fieldDisabled = false) => {
+    registerField: (field, defaultValue = null, fieldDisabled = false) => {
       if (!field) {
         return
       }

packages/standard-components/src/forms/Picker.svelte

@@ -57,7 +57,7 @@
             role="option"
             aria-selected="true"
             tabindex="0"
-            on:click={() => onSelectOption(undefined)}>
+            on:click={() => onSelectOption(null)}>
             <span class="spectrum-Menu-itemLabel">{placeholderOption}</span>
             <svg
               class="spectrum-Icon spectrum-UIIcon-Checkmark100 spectrum-Menu-checkmark spectrum-Menu-itemIcon"

packages/standard-components/src/forms/RelationshipField.svelte

@@ -77,7 +77,7 @@
 
   const toggleOption = id => {
     if (fieldSchema.relationshipType === "one-to-many") {
-      fieldApi.setValue([{ _id: id }])
+      fieldApi.setValue(id ? [{ _id: id }] : [])
     } else {
       if ($fieldState.value.find(option => option?._id === id)) {
         const filtered = $fieldState.value.filter(option => option?._id !== id)