Return "" and not "Unknown" from wxMSW wxGetCpuArchitectureName()

This is more consistent with the Unix version, and easier to test for
in the application code.

interface/wx/utils.h

@@ -995,6 +995,9 @@ bool wxIsPlatformLittleEndian();
     "arm64", or "i86pc". The name for the same CPU running on the same
     hardware can vary across operating systems.
 
+    The returned string may be empty if the CPU architecture couldn't be
+    recognized.
+
     @since 3.1.5
 */
 wxString wxGetCpuArchitectureName();

src/msw/utils.cpp

@@ -1297,7 +1297,7 @@ wxString wxGetCpuArchitectureName()
 
     case PROCESSOR_ARCHITECTURE_UNKNOWN:
     default:
-        return "Unknown";
+        return wxString();
     }
 }