fix(module:select): fix null value

[anranruye]

🤔 This is a ...

• New feature
• Bug fix
• Site / documentation update
• Demo update
• Component style update
• Bundle size optimization
• Performance optimization
• Refactoring
• Code style optimization
• Test Case
• Branch merge
• Other (about what?)

🔗 Related issue link

Fixes #1995
#1783

💡 Background and solution

📝 Changelog

Language	Changelog
🇺🇸 English	fix null value option
🇨🇳 Chinese

☑️ Self Check before Merge

⚠️ Please check all items below before review. ⚠️

• Doc is updated/provided or not needed
• Demo is updated/provided or not needed
• Changelog is provided or not needed

[github-actions]

[codecov]

Codecov Report

Merging #1996 (223fa8b) into master (ca3e2e1) will increase coverage by 1.91%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master    #1996      +/-   ##
==========================================
+ Coverage   25.38%   27.30%   +1.91%     
==========================================
  Files         488      511      +23     
  Lines       32098    24482    -7616     
  Branches        0      233     +233     
==========================================
- Hits         8148     6684    -1464     
+ Misses      23950    17762    -6188     
- Partials        0       36      +36     

Impacted Files	Coverage Δ
...ents/select/internal/DataSourceEqualityComparer.cs	57.14% <33.33%> (+7.14%)	⬆️
components/select/SelectOption.razor.cs	83.63% <100.00%> (+4.26%)	⬆️
components/core/Helpers/MemberPath/PathNode.cs	46.15% <0.00%> (-8.85%)	⬇️
components/core/Reflection/TypeDefined.cs	77.77% <0.00%> (-7.94%)	⬇️
components/input/InputGroup.razor.cs	87.50% <0.00%> (-6.95%)	⬇️
components/tabs/TabPane.razor	86.66% <0.00%> (-6.20%)	⬇️
...ponents/date-picker/types/DatePickerPlaceholder.cs	22.72% <0.00%> (-4.20%)	⬇️
components/input/TextArea.razor.cs	77.52% <0.00%> (-3.05%)	⬇️
...ponents/core/Component/Overlay/type/TriggerType.cs	81.25% <0.00%> (-2.97%)	⬇️
components/timeline/Timeline.razor.cs	63.21% <0.00%> (-2.24%)	⬇️
... and 423 more

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[anddrzejb]

Sorry to keep you waiting. I am a bit swamped.
I tested the changes using these reproductions of each of the issues:
#1783

<Select DataSource="@_list"
		@bind-Value="@_selectedValue1"		
		ValueName="@nameof(Person.Id)"
		LabelName="@nameof(Person.Name)"		
		Style="width:250px">
</Select>

@code {
    record Person(string? Id, string Name);   
    string? _selectedValue1 = null;

    List<Person> _list = new()
    {
        new Person(null, "John"),
        new Person("2", "Lucy"),
        new Person("3", "Jack"),
        new Person("4", "Emily")
    };         
}

#1995

<Select @bind-Value="UserAssignedId"
        TItemValue="string"
        TItem="string"
        DefaultActiveFirstOption="true"
        Style="width:250px">
    <SelectOptions>
        <SelectOption TItemValue="string" TItem="string" Value="null" Label="Not assigned" />
        <SelectOption TItemValue="string" TItem="string" Value="@("test")" Label="Test" />
    </SelectOptions>
</Select>

@code {
    string? UserAssignedId = null;
}

1. The Select component will throw exception if the value is null #1783 reproduction does not show label for the null value after init:
   

2. Changing back to null value does not show label. The null value has to be selected once more for it to show in the Select input box:
   

[anranruye]

@anddrzejb I think this is the issue tracked by #1587. Do you think we should also fix it here?

[anddrzejb]

Oh, you are correct, I forgot about that issue. It does seem to be directly connected with null value, that is why I thought it could be a part of this PR.

[anddrzejb]

@anranruye are you planning to fix the visual issues I mentioned above in this PR? If not, I will approve it.

[anranruye]

@anddrzejb Then let's close this one. I will fix that issue when I have time, in another pr.