Add back cervix value filter

lib/sympto/cervix.js

@@ -2,6 +2,7 @@ export default function (cycleDays, tempEvalEndIndex) {
   const notDetected = { detected: false }
   const cervixDays = cycleDays
     .filter(day => day.cervix && !day.cervix.exclude)
+    .filter(day => typeof day.cervix.opening === 'number' && typeof day.cervix.firmness === 'number')
 
   // we search for the day of cervix peak, which must:
   // * have fertile cervix values