Sparkline not working in server/client mode

[daviehh]

Describe the bug
Running directly, the sparkline command switch or keypress 'S' works fine both on server and on client; however, glances just quits when running in server/client mode when --sparkline is provided or pressing 'S'.

To Reproduce
Steps to reproduce the behavior:

1. Start Glances with the followings options 'glances -s' on server, and 'glances -c ...' on client
2. Press the key 'S'
3. glance quits; with the --debug switch on, the log shows

2021-06-18 18:10:58,525 -- DEBUG -- Keypressed (code: 83)
2021-06-18 18:10:58,579 -- CRITICAL -- 'NoneType' object is not iterable

Expected behavior
Show sparkines in server/client mode

Desktop (please complete the following information):

• OS: macos 11.4 (20F71) and Linux raspberrypi 5.10.42-v8+ (debian/raspbian)
• Glances Version [output of glances -V]: Glances v3.1.7 with PsUtil v5.8.0
• Glances logs file

2021-06-18 18:10:58,525 -- DEBUG -- Keypressed (code: 83)
2021-06-18 18:10:58,579 -- CRITICAL -- 'NoneType' object is not iterable

• Glances test [output of glances --issue] (only available with Glances 3.1.7 or higher)

=========================================================
Glances version 3.1.7 with PsUtil 5.8.0

alert [OK] 0.0000s []
amps [OK] 0.0000s []
cloud [OK] 0.0000s {}
connections [OK] 0.0000s {}
core [OK] 0.0000s {'phys': 4, 'log': 8}
cpu [OK] 0.0001s {'total': 14.7, 'user': 9.8, 'system': 4.9, 'idle': 85.3, 'nice': 0.0, 'ctx_switches': -1851, 'interrupts': -313, 'soft_interru
diskio [OK] 0.0008s [{'time_since_update': 1, 'disk_name': 'disk0', 'read_count': 19, 'write_count': 2, 'read_bytes': 217088, 'write_bytes': 8192,
docker [OK] 0.0002s []
folders [OK] 0.0000s []
fs [OK] 0.0006s [{'device_name': '/dev/disk1s1s1', 'fs_type': 'apfs', 'mnt_point': '/', 'size': 499963174912, 'used': 15498387456, 'free': 2416
gpu [OK] 0.0000s []
help [OK] 0.0000s None
ip [OK] 0.0110s {'address': '192.168.1.154', 'mask': '255.255.255.0', 'mask_cidr': 24, 'gateway': '192.168.1.1', 'public_address': '100.1.188.1
irq [OK] 0.0000s []
load [OK] 0.0000s {'min1': 1.84375, 'min5': 1.77197265625, 'min15': 1.87060546875, 'cpucore': 8}
mem [OK] 0.0001s {'total': 17179869184, 'available': 6934773760, 'percent': 59.6, 'used': 10245095424, 'free': 6934773760, 'active': 6128607232,
memswap [OK] 0.0001s {'total': 2147483648, 'used': 450363392, 'free': 1697120256, 'percent': 21.0, 'sin': 55576432640, 'sout': 200404992}
network [OK] 0.0014s [{'interface_name': 'lo0', 'time_since_update': 1, 'cumulative_rx': 426137600, 'rx': 0, 'cumulative_tx': 426137600, 'tx': 0, 'c
now [OK] 0.0000s 2021-06-18 18:18:13 EDT
percpu [OK] 0.0000s [{'key': 'cpu_number', 'cpu_number': 0, 'total': 36.6, 'user': 24.6, 'system': 11.9, 'idle': 63.4, 'nice': 0.0}, {'key': 'cpu_n
ports [OK] 0.0003s []
processcount [OK] 0.1046s {'total': 574, 'running': 574, 'sleeping': 0, 'thread': 1215, 'pid_max': 0}
processlist [OK] 0.0000s [{'cmdline': ['python', '[...]/bin/glances', '--issue'], 'memory_percent': 0.377273
psutilversion [OK] 0.0001s (5, 8, 0)
quicklook [OK] 0.2061s {'cpu': 14.7, 'percpu': [{'key': 'cpu_number', 'cpu_number': 0, 'total': 36.6, 'user': 24.6, 'system': 11.9, 'idle': 63.4, 'nic
raid [OK] 0.0001s {}
sensors [OK] 0.0006s [{'label': 'Battery', 'value': 97, 'unit': '%', 'type': 'battery', 'key': 'label'}]
smart [N/A]
system [OK] 0.0106s {'os_name': 'Darwin', 'hostname': '[].local', 'platform': '64bit', 'os_version': '10.16', 'hr_name': 'Darwin 10
uptime [OK] 0.0001s {'seconds': 91045}
wifi [OK] 0.0000s []

Additional context
Add any other context about the problem here.

[nicolargo]

Related to #1882