Avoid unguarded use of enum_exists (zircote#1486)

src/Processors/ExpandEnums.php

@@ -84,7 +84,7 @@ protected function expandSchemaEnum(Analysis $analysis): void
 
                 // transform each Enum cases into UnitEnum
                 foreach ($schema->enum as $enum) {
-                    if (is_string($enum) && enum_exists($enum)) {
+                    if (is_string($enum) && function_exists('enum_exists') && enum_exists($enum)) {
                         foreach ($enum::cases() as $case) {
                             $cases[] = $case;
                         }