Issue #26673: Protect IDLE from Linux fonts with reported default size 0.

Such fonts on Linux prevented the configuration dialog from opening.

Lib/idlelib/configHandler.py

@@ -720,7 +720,7 @@ class IdleConf:
                 actualFont = Font.actual(f)
                 family = actualFont['family']
                 size = actualFont['size']
-                if size < 0:
+                if size <= 0:
                     size = 10  # if font in pixels, ignore actual size
                 bold = actualFont['weight']=='bold'
         return (family, size, 'bold' if bold else 'normal')