Add handle empty pool

[kylinsoong]

Pool is empty

% ./f5-tmsh-generator.py config/f5config.3 test/request-pool-empty.txt
tmsh modify ltm pool ydbg_10.1.10.11_80_pool members add { :8080 }
tmsh modify ltm snatpool ydbg_10.1.10.11_snat members add { 10.1.10.108 10.1.10.109 }
tmsh save sys config

SNAT is empty

% ./f5-tmsh-generator.py config/f5config.3 test/request-snap-empty.txt 
tmsh modify ltm pool ydbg_10.1.10.11_80_pool members add { 10.1.20.13:8080 10.1.20.14:8080 10.1.20.15:8080 10.1.20.7:8080 10.1.20.28:8080 10.1.20.29:8080 10.1.20.30:8080 }
tmsh modify ltm snatpool ydbg_10.1.10.11_snat members add {  }
tmsh save sys config

[kylinsoong]

pool empty

% ./f5-tmsh-generator.py config/f5config.3 test/request-pool-empty.txt
tmsh modify ltm snatpool ydbg_10.1.10.11_snat members add { 10.1.10.108 10.1.10.109 }
tmsh save sys config

snatpool empty

% ./f5-tmsh-generator.py config/f5config.3 test/request-snap-empty.txt
tmsh modify ltm pool ydbg_10.1.10.11_80_pool members add { 10.1.20.13:8080 10.1.20.14:8080 10.1.20.15:8080 10.1.20.7:8080 10.1.20.28:8080 10.1.20.29:8080 10.1.20.30:8080 }
tmsh save sys config