Change current font only if new graphics font is successfully created

If new graphics font couldn't be created successfully there is no need
to reset current font by updating it to wxNullGraphicsFont.

Closes #17790.

interface/wx/dcgraph.h

@@ -10,6 +10,10 @@
 
     wxGCDC is a device context that draws on a wxGraphicsContext.
 
+    @remarks
+    If Direct2D is a renderer of underlying graphics context, only wxFont objects
+    representing TrueType fonts can be used in the font-related functions.
+
     @library{wxcore}
     @category{dc}