I get the same result (ARP entry without MAC address) when device with that IP address does not exist. For example when I try to ping non-existing IP address:
[admin@RBD52G] /ip arp> print
Flags: X - disabled, I - invalid, H - DHCP, D - dynamic, P - published, C - complete
# ADDRESS MAC-ADDRESS INTERFACE
0 DC 192.168.42.195 50:85:69:00:B6:8C vlan-42
1 DC 192.168.42.17 28:92:4A:26:F7:A3 vlan-42
2 DC 192.168.43.2 20:02:AF:7B:5F:AF vlan-42
3 DC 192.168.43.253 30:45:96:B8:74:87 vlan-42
4 DC 192.168.42.11 00:30:48:B4:4B:BE vlan-42
5 DC 192.168.42.10 AC:1F:6B:B0:26:BC vlan-42
6 DC 192.168.42.5 FC:75:16:74:D5:56 vlan-42
7 DC 192.168.42.142 24:C6:96:52:E6:A2 vlan-42
8 DC 192.168.42.145 84:98:66:7C:71:ED vlan-42
9 DC 192.168.1.1 64:6E:EA:07:8E:29 vlan-2
[admin@RBD52G] /ip arp> /ping 192.168.42.99
SEQ HOST SIZE TTL TIME STATUS
0 192.168.42.99 timeout
1 192.168.42.99 timeout
2 192.168.42.99 timeout
sent=3 received=0 packet-loss=100%
[admin@RBD52G] /ip arp> print
Flags: X - disabled, I - invalid, H - DHCP, D - dynamic, P - published, C - complete
# ADDRESS MAC-ADDRESS INTERFACE
0 DC 192.168.42.195 50:85:69:00:B6:8C vlan-42
1 DC 192.168.42.17 28:92:4A:26:F7:A3 vlan-42
2 DC 192.168.43.2 20:02:AF:7B:5F:AF vlan-42
3 DC 192.168.43.253 30:45:96:B8:74:87 vlan-42
4 DC 192.168.42.11 00:30:48:B4:4B:BE vlan-42
5 DC 192.168.42.10 AC:1F:6B:B0:26:BC vlan-42
6 DC 192.168.42.5 FC:75:16:74:D5:56 vlan-42
7 DC 192.168.42.142 24:C6:96:52:E6:A2 vlan-42
8 DC 192.168.42.145 84:98:66:7C:71:ED vlan-42
9 D 192.168.42.99 vlan-42