postgresql_extensible crash on null value

[aleszeleny]

Relevant telegraf.conf:

[[outputs.influxdb]]
  urls = ["http://localhost:8086"]

  [[inputs.postgresql_extensible.query]]
    sqlquery='''
      SELECT *
      FROM pg_catalog.pg_stat_activity
    '''
    version=100
    withdbname=false

System info:

Linux 4.4.0-1061-aws #70-Ubuntu SMP Fri May 25 21:47:34 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

telegraf --version
Telegraf v1.7.4 (git: release-1.7 578db7ef)

Steps to reproduce:

drop target measurement on influxdb side and

select have to capture at least one column with null value, can be simplified:

    sqlquery='''
      SELECT
        null as datname
      FROM pg_catalog.pg_stat_activity
'''

Expected behavior:

Insert null and did not crash

Actual behavior:

# /usr/bin/telegraf -config /etc/telegraf/fr_usd_dev-5432.conf
panic: interface conversion: interface {} is nil, not string

goroutine 16 [running]:
github.com/influxdata/telegraf/plugins/inputs/postgresql_extensible.(*Postgresql).accRow(0xc42019e120, 0xc420304cc1, 0x10, 0x17ac460, 0xc4201c8780, 0x17c9f20, 0xc42048dce0, 0xc4203b8c00, 0x2, 0x2, ...)
        /go/src/github.com/influxdata/telegraf/plugins/inputs/postgresql_extensible/postgresql_extensible.go:226 +0xe5e
github.com/influxdata/telegraf/plugins/inputs/postgresql_extensible.(*Postgresql).Gather(0xc42019e120, 0x17c9f20, 0xc42048dce0, 0x0, 0x0)
        /go/src/github.com/influxdata/telegraf/plugins/inputs/postgresql_extensible/postgresql_extensible.go:184 +0x40e
github.com/influxdata/telegraf/agent.gatherWithTimeout.func1(0xc42017c540, 0xc42040cf00, 0x17c9f20, 0xc42048dce0)
        /go/src/github.com/influxdata/telegraf/agent/agent.go:153 +0x47
created by github.com/influxdata/telegraf/agent.gatherWithTimeout
        /go/src/github.com/influxdata/telegraf/agent/agent.go:152 +0xd4

Additional info:

crash is not written to logfile.