devtool: modify: fix exception

devtool: modify: fix exception

Root Cause:
initial_revs is an empty dictionary and do not have "." key.

Traceback (most recent call last):
  File "scripts/devtool", line 349, in <module>
    ret = main()
  File "scripts/devtool", line 336, in main
    ret = args.func(args, config, basepath, workspace)
  File "scripts/lib/devtool/standard.py", line 922, in modify
    if not initial_revs["."]:
KeyError: '.'

Solution:
check key exists, then get its value.

Signed-off-by: Jamin Lin <jamin_lin@aspeedtech.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>