Fix PrettyJson colorize function

[NicolasMahe]

The PrettyJSON function was panicking because of a non initialized map.

panic: assignment to entry in nil map

goroutine 1 [running]:
github.com/mesg-foundation/core/utils/pretty.(*Pretty).ColorizeJSON(0xc0000ae680, 0xc0000be220, 0xc0000be200, 0xc0003c4500, 0x125, 0x140, 0x140, 0x125, 0xc0003c4500)
        /Users/nico/Developments/Go/src/github.com/mesg-foundation/core/utils/pretty/pretty.go:228 +0x2e4
github.com/mesg-foundation/core/utils/pretty.ColorizeJSON(0xc0000be220, 0xc0000be200, 0xc0003c4500, 0x125, 0x140, 0x140, 0x0, 0xc00077d280)
        /Users/nico/Developments/Go/src/github.com/mesg-foundation/core/utils/pretty/pretty.go:360 +0x63
github.com/mesg-foundation/core/commands.(*serviceDevCmd).runE(0xc0003f36e0, 0xc0000c9b00, 0xc00016eef0, 0x1, 0x1, 0x0, 0x0)
        /Users/nico/Developments/Go/src/github.com/mesg-foundation/core/commands/service_dev.go:98 +0xc85
github.com/mesg-foundation/core/commands.(*serviceDevCmd).runE-fm(0xc0000c9b00, 0xc00016eef0, 0x1, 0x1, 0x0, 0x0)
        /Users/nico/Developments/Go/src/github.com/mesg-foundation/core/commands/service_dev.go:37 +0x52
github.com/mesg-foundation/core/vendor/github.com/spf13/cobra.(*Command).execute(0xc0000c9b00, 0xc00016eec0, 0x1, 0x1, 0xc0000c9b00, 0xc00016eec0)
        /Users/nico/Developments/Go/src/github.com/mesg-foundation/core/vendor/github.com/spf13/cobra/command.go:746 +0x473
github.com/mesg-foundation/core/vendor/github.com/spf13/cobra.(*Command).ExecuteC(0xc00043efc0, 0x3, 0xc0001aaae0, 0xf)
        /Users/nico/Developments/Go/src/github.com/mesg-foundation/core/vendor/github.com/spf13/cobra/command.go:831 +0x2dc
github.com/mesg-foundation/core/vendor/github.com/spf13/cobra.(*Command).Execute(0xc00043efc0, 0x18d6271, 0x9)
        /Users/nico/Developments/Go/src/github.com/mesg-foundation/core/vendor/github.com/spf13/cobra/command.go:784 +0x2b
main.main()
        /Users/nico/Developments/Go/src/github.com/mesg-foundation/core/interface/cli/main.go:35 +0x25b
exit status 2