add comment about text field default (keystonejs#6942)

* add comment about default

* Update index.ts

Co-authored-by: Daniel Cousens <dcousens@users.noreply.github.com>
Co-authored-by: Thomas Walker <bladey@me.com>

packages/keystone/src/fields/types/text/index.ts

@@ -70,6 +70,7 @@ export const text =
       },
     };
 
+    // defaulted to false as a zero length string is preferred to null
     const isNullable = config.db?.isNullable ?? false;
 
     const fieldLabel = config.label ?? humanize(meta.fieldKey);