Change default Port from COM1 to nothing selected

Author: PaulStoffregen

build/shared/lib/preferences.txt

@@ -267,7 +267,8 @@ programmer = arduino:avrispmkii
 upload.using = bootloader
 upload.verify = true
 
-serial.port=COM1
+#default port is empty to prevent running AVRDUDE before Port selected (issue #7943)
+serial.port=
 serial.databits=8
 serial.stopbits=1
 serial.parity=N