fix: null values in min and max date with prevent-min-max-navigation stops navigation altogether (fixes #951)

Author: Jasenkoo

src/VueDatePicker/composables/validation.ts

@@ -94,7 +94,7 @@ export const useValidation = (props: PickerBasePropsType | AllPropsType) => {
 
     const validateMonthYearInRange = (month: number, year: number, isNext: boolean, preventNav: boolean): boolean => {
         let valid = false;
-        if (preventNav) {
+        if (preventNav && (propDates.value.minDate || propDates.value.maxDate)) {
             if (propDates.value.minDate && propDates.value.maxDate) {
                 valid = validateBothMinAndMax(month, year, isNext);
             } else if (propDates.value.minDate && validateMinDate(month, year)) {