fix(datepicker): emit null instead of undefined when the input is cleared

[mnkprs]

PR Checklist

Before creating new PR, please take a look at checklist below to make sure that you've done everything that needs to be done before we can merge it.

• read and followed the CONTRIBUTING.md guide.
• built and tested the changes locally.
• added/updated tests.
• N/A - no public API changes, so no API documentation updates needed.
• N/A - no new/changed demos needed; existing reactive-forms and signal-forms usage is unaffected other than the cleared-value type.

Root cause

BsDatepickerInputDirective and BsDaterangepickerInputDirective call their registered
onChange (the value accessor callback) with undefined whenever the input is cleared -
either by typing/deleting the text and blurring, or via the picker's own clear action, which
routes through the same bsValueChange subscription.

Angular's new Signal Forms (@angular/forms/signals) treats a model property that is
explicitly set to undefined as "this field no longer exists" and deletes the corresponding
FieldNode (FieldNodeStructure.computeChildrenMap). Once that happens, any further access to
the field - including FormField's own state computed (this.field()()) - throws:

TypeError: this.field(...) is not a function

...and the field can never be updated again, exactly as reported in #6832.

Change

Emit null instead of undefined from the CVA onChange callback whenever the datepicker or
daterangepicker input is cleared (both the native change event path and the picker-driven
bsValueChange path, for both directives). null is the conventional "empty" sentinel already
used by FormControl's own default, by this library's own timepicker (onChange(null) on an
invalid/empty entry), and by Angular Material's datepicker. Internal state (_value, the public
bsValue input, writeValue, validate) is unchanged; only the value handed to the form is
normalized at the point it's emitted.

Angular/components applied the identical fix for the same Signal Forms failure mode in
angular/components#33708 (chip listbox
deselection).

Tests

Added src/datepicker/testing/bs-datepicker-input.spec.ts covering both directives:

• Reactive forms: clearing the native input, and clearing via bsValue (the picker's own clear
  path), both now leave control.value as null rather than undefined, for bsDatepicker and
  bsDaterangepicker.
• Signal forms: binding [formField] to a bsDatepicker input and clearing it no longer throws
  this.field(...) is not a function, the field's model value becomes null, and the field stays
  alive so a subsequently selected date still reaches the model.

All new specs fail against the current development code (reproducing #6832, including the
exact error message) and pass with this change.

Validation:

• npx nx test datepicker: 57 passed, 2 skipped.
• npm test -- --parallel=2 --runInBand: all 24 projects passed.
• npx nx lint datepicker and npm run lint -- --parallel=2: all 46 projects passed, 0 errors
  (pre-existing, unrelated warnings only).
• npx nx build datepicker --configuration=production: passed.

Behavioral note

Consumers reading the cleared value from ngModel/FormControl/[formField] now receive
null instead of undefined. This only affects the "cleared" case; a date value is unaffected.

Fixes #6832