Remove Null Checker

[Noslin22]

Just removed the null check, now the field can be cleared by defining the selectedValue to null.

Before, if you tried to clear the selected value, you would get this error:

════════ Exception caught by widgets library ═══════════════════════════════════
The following _TypeError was thrown building SearchCitiesField(dependencies: [_LocalizationsScope-[GlobalKey#93cef]]):
Null check operator used on a null value

The relevant error-causing widget was:
    SearchCitiesField SearchCitiesField:file:///D:/Projects/Flutter/Fun%20Projects/planner_app/lib/ui/register/widgets/information_card.dart:78:38

When the exception was thrown, this was the stack:
#0      _SearchFieldState.didUpdateWidget (package:searchfield/src/searchfield.dart:678:52)
searchfield.dart:678
#1      StatefulElement.update (package:flutter/src/widgets/framework.dart:5803:55)
framework.dart:5803
#2      Element.updateChild (package:flutter/src/widgets/framework.dart:3941:15)
framework.dart:3941
#3      ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:5656:16)
framework.dart:5656
#4      Element.rebuild (package:flutter/src/widgets/framework.dart:5347:7)
framework.dart:5347
...

But now it works perfectly.

[maheshj01]

@Noslin22 thanks for the PR, I have pushed some updates to master branch please rebase the branch which should fix the test failure for this PR.

[maheshj01]

Thanks for the PR @Noslin22