Fix input warning

When the value isn't passed for rjsf the 'value' property isn't setted on 'material-ui' input and that show a warning message about 'uncontrolled' input.
Then when the values isn't passed for rjsf, the value will be empty value ('').
Resolves #21

Author: agustin107

src/PasswordWidget/PasswordWidget.tsx

@@ -42,7 +42,7 @@ const PasswordWidget = ({
         required={required}
         disabled={disabled || readonly}
         type="password"
-        value={value}
+        value={value ? value : ''}
         onFocus={_onFocus}
         onBlur={_onBlur}
         onChange={_onChange}

src/TextWidget/TextWidget.tsx

@@ -42,7 +42,7 @@ const TextWidget = ({
         required={required}
         disabled={disabled || readonly}
         name={name}
-        value={value}
+        value={value ? value : ''}
         onChange={_onChange}
         onBlur={_onBlur}
         onFocus={_onFocus}

src/UpDownWidget/UpDownWidget.tsx

@@ -41,7 +41,7 @@ const UpDownWidget = ({
         type="number"
         disabled={disabled || readonly}
         name={name}
-        value={value}
+        value={value ? value : ''}
         onChange={_onChange}
         onBlur={_onBlur}
         onFocus={_onFocus}