By this post I have found the option --get-value of the command nmcli. For an explanation of the option see the documentation of the command nmcli.
By this option I can specify what fields must be present in the output, and each field value is separated from the others by a colon character (':'). In this way, the command output shows the Wi-Fi network SSID complete with all leading and the trailing spaces are present in the SSID.
So the command nmcli can be used as showed below:
> nmcli --get-value SSID,SECURITY dev wifi list
An example of the output of the command is the followed (there are 2 columns SSID and SECURITY seprated by : char):
TPLINK-8853:WPA2
TPLINK-9910 :WPA2
GUESTS:
:WPA2 802.1X
...
The second line of the output is TPLINK-9910 :WPA2 and it is easy to get the SSID of the Wifi because it is located between the beginning of the line and the character : character.
In the output the fourth line (:WPA2 802.1X) shows an hidden wifi network (the SSID is empty).
This method solve also the problem of the leading spacespaces.
EDIT
As suggested by @Hans-Martin Mosner It is necessary to handle the case where the : character is contained in the SSID.
On my project the output of the nmcli command is processed by a script Python so by the split(':') method it is not difficult to get the SSID containing the character : possibly present.
Thank you to all