Fix crash in pillar if NTP FQDN is an empty string

Under some conditions e.g. when set from Monitor TUI NTP FQDN can be an
empty string which causes a crash

pillar;panic: runtime error: index out of range -1]

pillar;goroutine 170248 [running]:
pillar;github.com/lf-edge/eve/pkg/pillar/devicenetwork.ResolveWithSrcIP({0x0, 0x0}, {0xc002631950, 0x10, 0x10}, {0xc002631960, 0x10, 0x10})
pillar;  /pillar/devicenetwork/dns.go:79 +0x4d4
pillar;github.com/lf-edge/eve/pkg/pillar/devicenetwork.ResolveCacheWrap.func1({0x0, 0x0}, {0xc002631950, 0x10, 0x10}, {0xc002631960, 0x10, 0x10})
pillar;  /pillar/devicenetwork/dns.go:122 +0x1ca
pillar;github.com/lf-edge/eve/pkg/pillar/devicenetwork.ResolveWithPortsLambda.func1({0xc002631950, 0x10, 0x10}, {0xc002631960, 0x10, 0x10})
pillar;  /pillar/devicenetwork/dns.go:198 +0x17b
pillar;created by github.com/lf-edge/eve/pkg/pillar/devicenetwork.ResolveWithPortsLambda
pillar;  /pillar/devicenetwork/dns.go:182 +0x98a

Signed-off-by: Mikhail Malyshev <mike.malyshev@gmail.com>

pkg/pillar/cmd/zedrouter/networkinstance.go

@@ -105,6 +105,11 @@ func (z *zedrouter) attachNTPServersToPortConfigs(portConfigs []nireconciler.Por
 				ntpServers = append(ntpServers, ip)
 				continue
 			}
+
+			if len(strings.TrimSpace(ntpServer)) == 0 {
+				continue
+			}
+
 			z.pubSub.StillRunning(agentName, warningTime, errorTime)
 			dnsResponses, err := devicenetwork.ResolveWithPortsLambda(
 				ntpServer,