Text should still be centered when search bar height is less than 48 (#128068)

Fixes #127092

This fix can solve the alignment issue in `SearchBar` but we still need to investigate the root cause for the `TextField`. The 
text baseline of `TextField` doesn't change when the height is less than 48 and greater than 40. The problem should be related to the `minContainerHeight` which is 48 by default but the `contentHeight` has become smaller than this min value already. Setting `isDense`/`isCollapsed` to true gives the `minContainerHeight` a smaller number which is 0.0: https://github.com/flutter/flutter/blob/ff33555b239de4c17e7149db17eb70bcf5dee215/packages/flutter/lib/src/material/input_decorator.dart#L1086

Since [`isDense`](https://github.com/flutter/flutter/blob/ff33555b239de4c17e7149db17eb70bcf5dee215/packages/flutter/lib/src/material/input_decorator.dart#L3907) is used for the case where the text field has less vertical space, I just use this property in SearchBar.

https://github.com/flutter/flutter/assets/36861262/6ddc8e90-1b47-4dd5-9a57-59b86cafec6d

This is a demo to show the text baseline that doesn't change when we set the text field height under 48.
https://github.com/flutter/flutter/assets/36861262/ce2ee815-f1f5-493a-930e-0540a627bec8