fix(GuestForm): allow to create a guest without name more than once

[ShGKme]

Resolves

To reproduce - create a share with a guest without a name 2 times without page reloading.

• After creating a guest, the GuestForm modal is reset.
• During reset, values are set null instead of initial empty string ''
• Then null is sent to the server that expects a string which causes an error

[ShGKme]

/backport to stable28

[ShGKme]

/backport! to stable2.5

[ShGKme]

/backport to stable2.5