Fix leading slash handling for flat format

[earies]

Handle leading slash for flat format when origin is not included in prefix or
prefix is non-existant

Prior behavior:

$ gnmic -a r1:32767 -u user -p pass --insecure subscribe -e proto --format flat --mode once --stream-mode sample --path openconfig:/system/state
openconfig:/system/state/boot-time: 1760805556938250028
openconfig:/system/state/current-datetime: 2025-10-18T19:04:40Z
openconfig:/system/state/domain-name:
openconfig:/system/state/hostname: r1
openconfig:/system/state/software-version: 26.1I20251016234041-_cd-builder-1
openconfig:/system/state/up-time: 8719716420835

openconfig:system/state/login-banner:

openconfig:system/state/motd-banner: --- JUNOS 26.1I20251016234041-_cd-builder-1 Linux (none) 5.15.164-10.22.33.18-yocto-standard-juniper-17166-g088c19cd4948 #1 SMP PREEMPT Mon Oct 6 04:26:49 UTC 2025 x86_64 x86_64 x86_64 GNU/Linux

openconfig:system/state/last-configuration-timestamp: 1760805868949826900

When a prefix is included in the Notification, a leading slash is emitted

update:  {
  timestamp:  1760812704326423058
  prefix:  {
    origin:  "openconfig"
  }
  update:  {
    path:  {
      elem:  {
        name:  "system"
      }
      elem:  {
        name:  "state"
      }
      elem:  {
        name:  "hostname"
      }
    }
    val:  {
      string_val:  "r1"
    }
  }
  ...

However, when the Notification does not include a prefix and/or the origin is
excluded from the prefix Path, a leading slash is omitted

update:  {
  timestamp:  1760812704788924249
  update:  {
    path:  {
      origin:  "openconfig"
      elem:  {
        name:  "system"
      }
      elem:  {
        name:  "state"
      }
      elem:  {
        name:  "login-banner"
      }
    }
    val:  {
      string_val:  ""
    }
  }
}

Behavior post fix:

$ ./gnmic -a r1:32767 -u user -p pass --insecure subscribe -e proto --format flat --mode once --stream-mode sample --path openconfig:/system/state
openconfig:/system/state/boot-time: 1760805556938250028
openconfig:/system/state/current-datetime: 2025-10-18T19:16:34Z
openconfig:/system/state/domain-name:
openconfig:/system/state/hostname: r1
openconfig:/system/state/software-version: 26.1I20251016234041-_cd-builder-1
openconfig:/system/state/up-time: 9429717100519

openconfig:/system/state/login-banner:

openconfig:/system/state/motd-banner: --- JUNOS 26.1I20251016234041-_cd-builder-1 Linux (none) 5.15.164-10.22.33.18-yocto-standard-juniper-17166-g088c19cd4948 #1 SMP PREEMPT Mon Oct 6 04:26:49 UTC 2025 x86_64 x86_64 x86_64 GNU/Linux

openconfig:/system/state/last-configuration-timestamp: 1760805868949826900

Without origin population either in Notification prefix or Update

$ ./gnmic -a eos1:6030 -u admin -p admin --insecure subscribe -e proto --format flat --mode once --stream-mode sample --path /system/state
/system/state/boot-time: 1760811567867886066

/system/state/current-datetime: 2025-10-18T19:18:11Z

/system/state/hostname: eos1

/system/state/last-configuration-timestamp: 1760811955880948250

/system/state/software-version: 4.35.0F