[@mantine/dates] prevent endless loop in getEndOfWeek

[sjroesink]

Calling getEndOfWeek with an invalid date now causes an endless loop (getEndOfWeek("", "") for instance).
See https://codesandbox.io/p/sandbox/priceless-flower-8v9mhl?file=%2Fsrc%2FApp.js%3A7%2C5-7%2C17

This function is at least also called from DateInput. Simply setting an invalid value there also causes the loop after clicking the input:

<DateInput value={'invalid'} />

[rtivital]

Can you give an example of code which causes an issue for you?

[sjroesink]

Can you give an example of code which causes an issue for you?

@rtivital I updated the description.