[iOS] Simplify the code used to set the same font size for Editor/Ent…

…ry. (#13150)

PR #13140 landed with a property
implementation that is more complicatd than needed (also a little
unsafe).

Changes in this PR:

1. Use a nullable nfloat to ensure that we do not use a magic number as
the default value of the variable.
2. Simplify the if nests. There is no needed to have that many nested
code to achiecve the same. This implementation does the same with a much
simpler approach.

Please follow the logic of the changes in the comment review I gave to
the original PR:
#13140 (review)

src/Core/src/Platform/iOS/MauiTextView.cs

@@ -10,7 +10,7 @@ namespace Microsoft.Maui.Platform
 	public class MauiTextView : UITextView
 	{
 		readonly UILabel _placeholderLabel;
-		nfloat _defaultPlaceholderSize = -1;
+		nfloat? _defaultPlaceholderSize;
 
 		public MauiTextView()
 		{
@@ -176,18 +176,9 @@ void ShouldCenterVertically()
 
 		void UpdatePlaceholderFontSize(UIFont? value)
 		{
-			if (value != null)
-			{
-				if (_defaultPlaceholderSize == -1)
-				{
-					_defaultPlaceholderSize = _placeholderLabel.Font.PointSize;
-				}
-				_placeholderLabel.Font = _placeholderLabel.Font.WithSize(value.PointSize);
-			}
-			else if (_defaultPlaceholderSize != -1)
-			{
-				_placeholderLabel.Font = _placeholderLabel.Font.WithSize(_defaultPlaceholderSize);
-			}
+			_defaultPlaceholderSize ??= _placeholderLabel.Font.PointSize;
+			_placeholderLabel.Font = _placeholderLabel.Font.WithSize (
+				value?.PointSize ?? _defaultPlaceholderSize.Value);
 		}
 	}
-}
+}