[nurullahturkoglu]

Fixes #609

Since #577, MudInputExtended renders the end adornment only when HasAdornmentEnd is true. MudComboBox passed HasAdornmentEnd="@(AdornmentEnd != null)", so the default open/close icon was never rendered unless a custom AdornmentEnd was set. It now passes true, like MudSelectExtended, since the combobox always fills that slot with either the custom adornment or the default icon.

Added ComboBox_OpenCloseIconTest. It checks that the ArrowDropDown icon is rendered and changes to ArrowDropUp when the menu opens. It fails without the fix, and the rest of the tests pass.

I also checked the first example on the docs site (Blazor Server, Chrome). Without the change there is no icon. With it, the icon is back and flips when the menu opens.

Before:

After:

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.30%. Comparing base (b1e5a3f) to head (a99baf8).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #652   +/-   ##
=======================================
  Coverage   67.30%   67.30%           
=======================================
  Files         117      117           
  Lines        8487     8487           
  Branches     1498     1498           
=======================================
  Hits         5712     5712           
+ Misses       2243     2242    -1     
- Partials      532      533    +1     

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