allow data_dict to exclude drawer_name

[justindthomas]

What I did

I adjusted fanshow to accommodate a missing drawer_name key in the data_dict.

How I did it

On my Dell N3248TE-ON, if only one of my PSUs is active, show platform fan generates a Python error due to a missing DRAWER_FIELD_NAME (set to drawer_name earlier) value in the data_dict that is built from the STATE_DB. I'm not sure if this is a peculiarity of the Dell unit or if it's more common. If I should adjust something in the Dell platform scripts instead, let me know.

How to verify it

show platform fan no longer generates a Python exception when only one PSU is active.

Previous command output (if the output of a command-line utility has changed)

root@sonic:~# show platform fan
Traceback (most recent call last):
  File "/usr/local/bin/fanshow", line 85, in <module>
    fanShow.show()
  File "/usr/local/bin/fanshow", line 75, in show
    table.append((data_dict[DRAWER_FIELD_NAME], data_dict[LED_STATUS_FIELD_NAME], name, speed, data_dict[DIRECTION_FIELD_NAME], presence, status,
KeyError: 'drawer_name'

New command output (if the output of a command-line utility has changed)

jdt@sonic:~$ show platform fan
  Drawer    LED            FAN    Speed    Direction    Presence    Status          Timestamp
--------  -----  -------------  -------  -----------  ----------  --------  -----------------
FanTray1    N/A  FanTray1-Fan1      32%       intake     Present        OK  20240112 01:40:13
FanTray2    N/A  FanTray2-Fan1      37%       intake     Present        OK  20240112 01:40:13
FanTray3    N/A  FanTray3-Fan1      34%       intake     Present        OK  20240112 01:40:13
     N/A    N/A       PSU1 Fan      N/A          N/A     Present        OK  20240112 01:40:11
     N/A    N/A       PSU2 Fan      19%       intake     Present        OK  20240112 01:40:14

It seems like maybe more work is required on this since Status should probably not say OK. But at least it doesn't throw an exception now.