provider: Handle empty nameservers

provider: Handle empty nameservers

If the VPN server doesn't set any DNS entries provider->nameservers is NULL.
When we change the state of the provider to CONNMAN_PROVIDER_STATE_READY
we shouldn't do any string operations on the NULL pointer namerservers.

(gdb) bt full
No symbol table info available.
        type = CONNMAN_ELEMENT_TYPE_IPV4
        nameservers = 0x0
        value = 0x0
        list = 0x1 = {<error reading variable list (Cannot access memory at address 0x1)>
        err = 0
        element = 0x6a5110
        name = 0x6a5550 "vpn0"
        second_ns = 0x69f1c0 "\004"
        service = 0x6a2630
No locals.

Fixes BMC#12508

Reported by Jeff Zheng <jeff.zheng@intel.com>