fix build_args map entries value may null

[laiiihz]

this distribute_options.yaml

...
  - name: android
    jobs:    
      - name: arm
        package:
          platform: android
          target: apk
          build_args:
            flavor: arm
            obfuscate:
            split-debug-info: .
...

will cause type 'Null' is not a subtype of type 'String' issue

type 'Null' is not a subtype of type 'String'
#0      AppBuilder.build (package:flutter_app_builder/src/app_builder.dart:38:37)
#1      FlutterAppBuilder.build (package:flutter_app_builder/src/flutter_app_builder.dart:42:26)
#2      FlutterDistributor.package (package:flutter_distributor/src/flutter_distributor.dart:145:40)
#3      FlutterDistributor.release (package:flutter_distributor/src/flutter_distributor.dart:302:49)

[lijy91]

LGTM