fix(material/select): incorrect default typeahead debounce time (#21253)

In 17a9ad0 so logic was added that was
setting the typeahead debounce interval to 0 if it isn't specified globally.
This is incorrect, because we want it to stay undefined so the list key manager
can fall back to its default interval.

Fixes #21242.

(cherry picked from commit ab01f38)

Author: crisbeto

src/material/select/select.ts

@@ -427,7 +427,7 @@ export abstract class _MatSelectBase<C> extends _MatSelectMixinBase implements A
   set typeaheadDebounceInterval(value: number) {
     this._typeaheadDebounceInterval = coerceNumberProperty(value);
   }
-  private _typeaheadDebounceInterval = this._defaultOptions?.typeaheadDebounceInterval ?? 0;
+  private _typeaheadDebounceInterval: number;
 
   /**
    * Function used to sort the values in a select in multiple mode.
@@ -504,6 +504,12 @@ export abstract class _MatSelectBase<C> extends _MatSelectMixinBase implements A
       this.ngControl.valueAccessor = this;
     }
 
+    // Note that we only want to set this when the defaults pass it in, otherwise it should
+    // stay as `undefined` so that it falls back to the default in the key manager.
+    if (_defaultOptions?.typeaheadDebounceInterval != null) {
+      this._typeaheadDebounceInterval = _defaultOptions.typeaheadDebounceInterval;
+    }
+
     this._scrollStrategyFactory = scrollStrategyFactory;
     this._scrollStrategy = this._scrollStrategyFactory();
     this.tabIndex = parseInt(tabIndex) || 0;