Check existence of fieldOptions in validators

In some validator functions, we were checking the existence
of min and max values in the field schema using
!isNil(field.fieldOptions.min) and !isNil(field.fieldOptions.max).

However, some fields might have very simple configuration which
doesn't include fieldOptions object, meaning that this code
will throw an error

```js
TypeError: Cannot read property 'min' of undefined
```
which vue throws as something like

```js
vue.esm.js:629 [Vue warn]: Error in event handler for "validate-fields": "TypeError: Cannot read property 'min' of undefined"
```
To guard against this, we need to first make sure that
fieldOptions also exists before we try to check whether attributes in it
are defined.

This was already present in `number` validator, but was missing in
`date`, `array` and `string`.

Author: Cyprian Muchoki

src/utils/validators.js

@@ -110,11 +110,11 @@ const validators = {
 
 		let err = [];
 		if (isString(value)) {
-			if (!isNil(field.fieldOptions.min) && value.length < field.fieldOptions.min) {
+			if (!isNil(field.fieldOptions) && !isNil(field.fieldOptions.min) && value.length < field.fieldOptions.min) {
 				err.push(msg(messages.textTooSmall, value.length, field.fieldOptions.min));
 			}
 
-			if (!isNil(field.fieldOptions.max) && value.length > field.fieldOptions.max) {
+			if (!isNil(field.fieldOptions) && !isNil(field.fieldOptions.max) && value.length > field.fieldOptions.max) {
 				err.push(msg(messages.textTooBig, value.length, field.fieldOptions.max));
 			}
 		} else {
@@ -136,11 +136,11 @@ const validators = {
 		}
 
 		if (!isNil(value)) {
-			if (!isNil(field.fieldOptions.min) && value.length < field.fieldOptions.min) {
+			if (!isNil(field.fieldOptions) && !isNil(field.fieldOptions.min) && value.length < field.fieldOptions.min) {
 				return [msg(messages.selectMinItems, field.fieldOptions.min)];
 			}
 
-			if (!isNil(field.fieldOptions.max) && value.length > field.fieldOptions.max) {
+			if (!isNil(field.fieldOptions) && !isNil(field.fieldOptions.max) && value.length > field.fieldOptions.max) {
 				return [msg(messages.selectMaxItems, field.fieldOptions.max)];
 			}
 		}
@@ -157,14 +157,14 @@ const validators = {
 
 		let err = [];
 
-		if (!isNil(field.fieldOptions.min)) {
+		if (!isNil(field.fieldOptions) && !isNil(field.fieldOptions.min)) {
 			let min = new Date(field.fieldOptions.min);
 			if (m.valueOf() < min.valueOf()) {
 				err.push(msg(messages.dateIsEarly, fecha.format(m), fecha.format(min)));
 			}
 		}
 
-		if (!isNil(field.fieldOptions.max)) {
+		if (!isNil(field.fieldOptions) && !isNil(field.fieldOptions.max)) {
 			let max = new Date(field.fieldOptions.max);
 			if (m.valueOf() > max.valueOf()) {
 				err.push(msg(messages.dateIsLate, fecha.format(m), fecha.format(max)));