[InputBase] Allow undefined in onBlur event type

[unclebay143]

Fixes #45982: Add undefined to onBlur event type in InputBase

Problem:
InputBase calls onBlur with undefined in some cases (e.g., when disabled), but the type didn’t reflect this.

Solution:
Updated the type to:

onBlur?: React.FocusEventHandler<HTMLInputElement | HTMLTextAreaElement> | undefined;

• I have followed (at least) the PR section of the contributing guide.

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[ZeeshanTamboli]

@unclebay143 Thanks for opening the PR, but the solution isn't correct.

Only the event parameter type should be undefined not the onBlur prop. So, we need to change it to:

- onBlur?: React.FocusEventHandler<HTMLInputElement | HTMLTextAreaElement>;
+   onBlur?: {
+   bivarianceHack(
+    event: React.FocusEvent<HTMLInputElement | HTMLTextAreaElement> | undefined,
+   ): void;
+ }['bivarianceHack'];

We can't do:

onBlur?: React.EventHandler<React.FocusEvent<HTMLInputElement | HTMLTextAreaElement> | undefined>;

because SyntheticEvent doesn't support undefined.

But this would be a breaking change because now the developer would need to access the event properties using the optional chaining operator event?.target.

I'll add a comment in #45982 that this is a breaking change so we won't be able to release this in a minor or patch version.