FIX Treat value not in SingleSelectField options as blank

This makes the react dropdown fields behave more like the entwine ones when the DB value is not in the set of dropdown options.

src/Forms/SingleSelectField.php

@@ -53,12 +53,13 @@ public function getSchemaDataDefaults()
     public function getDefaultValue()
     {
         $value = $this->Value();
+        $validValues = $this->getValidValues();
         // assign value to field, such as first option available
-        if ($value === null) {
+        if ($value === null || !in_array($value, $validValues)) {
             if ($this->getHasEmptyDefault()) {
                 $value = '';
             } else {
-                $values = $this->getValidValues();
+                $values = $validValues;
                 $value = array_shift($values);
             }
         }