Initialize the last fec ber computed values if not found (sonic-net#3621)

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**What I did**

`SAI_PORT_STAT_IF_FEC_CORRECTED_BITS_last` and `SAI_PORT_STAT_IF_FEC_NOT_CORRECTABLE_FARMES_last` counter values could be NOT present in redis, so initialize these values to ZERO

**Why I did it**
To prevent following error seen during warm boot

```
root@sonic:/# redis-cli --eval ./usr/share/swss/port_rates.lua oid:0x100000000000a , 2 COUNTERS 10
(error) ERR user_script:204: attempt to concatenate local 'fec_corr_bits_last' (a boolean value) script: b46cd0b446729aab6bbd20e0d4aa977e3da8f5f1, on @user_script:204.
root@sonic:/# redis-cli --eval ./usr/share/swss/port_rates.lua oid:0x100000000000a , 2 COUNTERS 10
(error) ERR user_script:204: attempt to concatenate local 'fec_corr_bits_last' (a boolean value) script: b46cd0b446729aab6bbd20e0d4aa977e3da8f5f1, on @user_script:204.
root@sonic:/# redis-cli --eval ./usr/share/swss/port_rates.lua oid:0x100000000000a , 2 COUNTERS 10
(error) ERR user_script:206: attempt to perform arithmetic on local 'fec_corr_bits_last' (a boolean value) script: cb98161ae9433f9bf8265cffd7d2eaaf6ac9282f, on @user_script:206.
root@sonic:/#
```

**How I verified it**
After fix the last computer FEC ber values are stored in redis DB

```
root@sonic:/# redis-cli --eval ./usr/share/swss/port_rates.lua oid:0x100000000000a , 2 COUNTERS 10
1) "0.18"
2) "0.82"
3) "10"
4) "DONE"
root@str2-7050cx3-acs-14:/# redis-cli -n 2 hgetall "RATES:oid:0x100000000000a"
 1) "SAI_PORT_STAT_IF_IN_UCAST_PKTS_last"
 2) "0"
 3) "SAI_PORT_STAT_IF_IN_NON_UCAST_PKTS_last"
 4) "0"
 5) "SAI_PORT_STAT_IF_OUT_UCAST_PKTS_last"
 6) "0"
 7) "SAI_PORT_STAT_IF_OUT_NON_UCAST_PKTS_last"
 8) "0"
 9) "SAI_PORT_STAT_IF_IN_OCTETS_last"
10) "0"
11) "SAI_PORT_STAT_IF_OUT_OCTETS_last"
12) "0"
13) "RX_BPS"
14) "0"
15) "RX_PPS"
16) "0"
17) "TX_BPS"
18) "0"
19) "TX_PPS"
20) "0"
21) "SAI_PORT_STAT_IF_FEC_CORRECTED_BITS_last"
22) "0"
23) "SAI_PORT_STAT_IF_FEC_NOT_CORRECTABLE_FARMES_last"
24) "0"
25) "FEC_PRE_BER"
26) "0"
27) "FEC_POST_BER"
28) "0"
root@str2-7050cx3-acs-14:/#
```

**Details if related**

Author: mssonicbld

orchagent/port_rates.lua

@@ -200,14 +200,17 @@ local function compute_rate(port)
             local fec_corr_bits_last = redis.call('HGET', rates_table_name .. ':' .. port, 'SAI_PORT_STAT_IF_FEC_CORRECTED_BITS_last')
             local fec_uncorr_frames_last = redis.call('HGET', rates_table_name .. ':' .. port, 'SAI_PORT_STAT_IF_FEC_NOT_CORRECTABLE_FARMES_last')
 
+            -- Initialize to 0 if last counter values does not exist (during first boot for eg)
+            fec_corr_bits_last = tonumber(fec_corr_bits_last) or 0
+            fec_uncorr_frames_last = tonumber(fec_uncorr_frames_last) or 0
+
             local serdes_rate_total = lanes_count * serdes_speed * delta / 1000
 
             fec_corr_bits_ber_new = (fec_corr_bits - fec_corr_bits_last) / serdes_rate_total
             fec_uncorr_bits_ber_new = (fec_uncorr_frames - fec_uncorr_frames_last) * rs_average_frame_ber  / serdes_rate_total
         else
             logit("FEC counters or lane info not found on " .. port)
         end
-
     else
         redis.call('HSET', state_table, 'INIT_DONE', 'COUNTERS_LAST')
     end