Fix empty invoice creation attempts

I stumbled across this while checking if anons can edit their items.

I monkey patched the code to make it possible (so they can see the 'edit' button) and tried to edit an item but I got this error:

  Variable "$amount" of required type "Int!" was not provided.

I fixed this even though this function should never be called without an amount anyway. It will return a sane error in that case now.

components/fee-button.js

@@ -140,6 +140,10 @@ export function EditFeeButton ({ paidSats, hadImgLink, hasImgLink, ChildButton,
   const addImgLink = hasImgLink && !hadImgLink
   const cost = (addImgLink ? paidSats * 9 : 0) + Number(boost)
 
+  useEffect(() => {
+    formik.setFieldValue('cost', cost)
+  }, [cost])
+
   const show = alwaysShow || !formik?.isSubmitting
   return (
     <div className='d-flex align-items-center'>

components/invoice.js

@@ -215,7 +215,7 @@ export const useInvoiceable = (fn, options = defaultOptions) => {
     if (!me && options.requireSession) {
       throw new Error('you must be logged in')
     }
-    if (me && !options.forceInvoice) {
+    if (!amount || (me && !options.forceInvoice)) {
       try {
         return await fn(amount, ...args)
       } catch (error) {