wpf: Add a TerminalControlSize emptiness check (#8906)

Watson reports show that an "ArgumentException" is being thrown due to `renderSize`
not being valid. Added a check for renderSize before attempting to resize.

src/cascadia/WpfTerminalControl/TerminalContainer.cs

@@ -138,7 +138,7 @@ internal void SetTheme(TerminalTheme theme, string fontFamily, short fontSize)
 
             NativeMethods.TerminalSetTheme(this.terminal, theme, fontFamily, fontSize, (int)dpiScale.PixelsPerInchX);
 
-            if (!this.RenderSize.IsEmpty)
+            if (!this.RenderSize.IsEmpty && !this.TerminalControlSize.IsEmpty)
             {
                 this.Resize(this.TerminalControlSize);
             }