Fix changing --git-builder

[brmzkw]

If an application has a buildpack builder, without any settings set, the command koyeb service update theapp/service --git-builder docker doesn't change the builder type.

here we should add:

|| (flags.Lookup("git-builder").Changed && builder == "docker") {

and here we should check if source.GetBuildpack returns a struct with only empty fields.

We also need to update here to add:

		(flags.Lookup("git-builder").Changed && builder == "buildpack") {

and just below, also check if the struct returned by GetDocker() is empty.