Clear invalid state when focused date is changed

A focused date is always expected to be valid (the user should not be able to focus invalid dates).

src/vaadin-date-picker-mixin.html

@@ -488,6 +488,7 @@
       this.__userInputOccurred = true;
       if (!this._ignoreFocusedDateChange && !this._noInput) {
         this._inputValue = focusedDate ? formatDate(Vaadin.DatePickerHelper._extractDateParts(focusedDate)) : '';
+        this.invalid = false;
       }
     }