Fix some logic of config

[jonyhy96]

Description

1. change second part of WatchdogConfig.Process's response to nil rather than an empty slice.
2. change the logic of mapEnv and ignore empty value environment

Motivation and Context

1. original Process will return a 0 length empty slice, By changing that into a nil will prevent unnecessary alloc.
2. empty value environment is useless for building a config.

How Has This Been Tested?

go test -run Test_mapEnv

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have signed-off my commits with git commit -s
• I have added tests to cover my changes.
• All new and existing tests passed.

[jonyhy96]

/cc @alexellis