bug(balancer): config with empty VS list can not be applied

[egnees]

Currently, applying config with empty VS list gives error, which is unexpected behavior.

Config example:

packet_handler:
  vs: []
  # Source addresses for the balancer
  source_address_v4: "IPv4 address"
  source_address_v6: "IPv6 address"

  # Addresses for decapsulation
  decap_addresses:
    - "IPv4 address"
    - "IPv6 address"

  # Session timeouts (in seconds)
  sessions_timeouts:
    tcp_syn_ack: 10
    tcp_syn: 10
    tcp_fin: 10
    tcp: 60
    udp: 30
    default: 60

# State management configuration
state:
  # Session table configuration
  session_table:
    capacity: 1000000  # Maximum concurrent sessions
    max_load_factor: 0.75  # Trigger resize at 75% capacity

  # WLC (Weighted Least Connections) configuration
  # Used when vs.flags.wlc = true
  wlc:
    power: 10  # Adjustment aggressiveness
    max_weight: 1000  # Maximum weight after adjustment

  # Periodic refresh for session counting and weight adjustment
  # Set to 0 to disable
  refresh_period_ms: 0