[bug fix] Fix panic when there is only one entity in dac

[cffls]

rand.Intn can't take 0 as an argument. This PR fixes a panic when there is only one entity in data committee.

[zkevm-node-sequence-sender-001] panic: invalid argument to Intn
[zkevm-node-sequence-sender-001]
[zkevm-node-sequence-sender-001] goroutine 1 [running]:
[zkevm-node-sequence-sender-001] math/rand.(*Rand).Intn(0x0?, 0x0?)
[zkevm-node-sequence-sender-001] 	/usr/local/go/src/math/rand/rand.go:180 +0x4c
[zkevm-node-sequence-sender-001] math/rand.Intn(0xc00013aaa0?)
[zkevm-node-sequence-sender-001] 	/usr/local/go/src/math/rand/rand.go:453 +0x25
[zkevm-node-sequence-sender-001] github.com/0xPolygonHermez/zkevm-sequence-sender/dataavailability/datacommittee.(*Backend).Init(0xc00013aaa0)
[zkevm-node-sequence-sender-001] 	/src/dataavailability/datacommittee/datacommittee.go:85 +0x7a
[zkevm-node-sequence-sender-001] github.com/0xPolygonHermez/zkevm-sequence-sender/dataavailability.New(...)
[zkevm-node-sequence-sender-001] 	/src/dataavailability/dataavailability.go:20

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
No data about Duplication

See analysis details on SonarCloud

[Sharonbc01]

Tag @xavier-romero