handle empty lineage entries gracefully

[kevgo]

Steps to reproduce

• have a lineage entry that is empty in either the branch or parent name
• run any Git Town command

expected behavior: it deletes the invalid lineage entries

actual behavior: it crashes

Command output

panic: local branch names cannot be empty

goroutine 1 [running]:
github.com/git-town/git-town/v14/src/git/gitdomain.NewLocalBranchName(...)
        C:/Users/kevin/d/git-town/src/git/gitdomain/local_branch_name.go:16
github.com/git-town/git-town/v14/src/git/gitdomain.NewLocalBranchNames(...)
        C:/Users/kevin/d/git-town/src/git/gitdomain/local_branch_names.go:16
github.com/git-town/git-town/v14/src/git/gitdomain.ParseLocalBranchNames({0xc000268168?, 0xbc00000001756ca0?})
        C:/Users/kevin/d/git-town/src/git/gitdomain/local_branch_names.go:28 +0xc5
github.com/git-town/git-town/v14/src/config/gitconfig.AddKeyToPartialConfig({0x166ccdb, 0x1b}, {0xc000268168, 0xe}, 0xc000114f30)
        C:/Users/kevin/d/git-town/src/config/gitconfig/access.go:103 +0xe1f
github.com/git-town/git-town/v14/src/config/gitconfig.(*Access).load(_, _, _)
        C:/Users/kevin/d/git-town/src/config/gitconfig/access.go:256 +0x553
github.com/git-town/git-town/v14/src/config/gitconfig.(*Access).LoadLocal(...)
        C:/Users/kevin/d/git-town/src/config/gitconfig/access.go:33
github.com/git-town/git-town/v14/src/execute.OpenRepo({0x0, 0x0, 0x1, 0x1, 0x0, 0x0})
        C:/Users/kevin/d/git-town/src/execute/open_repo.go:47 +0x307
github.com/git-town/git-town/v14/src/cmd.executeSwitch(0x0, 0x0?)
        C:/Users/kevin/d/git-town/src/cmd/switch.go:38 +0x5f
github.com/git-town/git-town/v14/src/cmd.switchCmd.func1(0xc0001cc800?, {0x1cf2260?, 0x4?, 0x165a9a9?})
        C:/Users/kevin/d/git-town/src/cmd/switch.go:29 +0x47
github.com/spf13/cobra.(*Command).execute(0xc0001bdb00, {0x1cf2260, 0x0, 0x0})
        C:/Users/kevin/go/pkg/mod/github.com/spf13/cobra@v1.8.0/command.go:983 +0xabc
github.com/spf13/cobra.(*Command).ExecuteC(0xc000004300)
        C:/Users/kevin/go/pkg/mod/github.com/spf13/cobra@v1.8.0/command.go:1115 +0x3ff
github.com/spf13/cobra.(*Command).Execute(...)
        C:/Users/kevin/go/pkg/mod/github.com/spf13/cobra@v1.8.0/command.go:1039
github.com/git-town/git-town/v14/src/cmd.Execute()
        C:/Users/kevin/d/git-town/src/cmd/core.go:38 +0x619

Additional information

No response