Make is_flat_memory more robust (sonic-net#491)

- Description
Currently, is_flat_memory could return 3 different values:

True when the module is flat memory
False when the module is not flat memory
None when the EEPROM read returns None or empty bytearray
When it returns None, we should not assume that the module is NOT a flat memory. Instead, it would be safer to treat it as a flat memory.

- Motivation and Context
This PR is to make is_flat_memory only returns two state, True or False. And if EEPROM reading returns None or empty bytearray, we should treat the module as a flat memory.

- How Has This Been Tested?
Manual test

sonic_platform_base/sonic_xcvr/api/public/cmis.py

@@ -368,7 +368,7 @@ def get_voltage(self):
         return float("{:.3f}".format(voltage))
 
     def is_flat_memory(self):
-        return self.xcvr_eeprom.read(consts.FLAT_MEM_FIELD)
+        return self.xcvr_eeprom.read(consts.FLAT_MEM_FIELD) is not False
 
     def get_temperature_support(self):
         return not self.is_flat_memory()

sonic_platform_base/sonic_xcvr/api/public/sff8436.py

@@ -273,7 +273,7 @@ def set_power_override(self, power_override, power_set):
         return ret
 
     def is_flat_memory(self):
-        return self.xcvr_eeprom.read(consts.FLAT_MEM_FIELD)
+        return self.xcvr_eeprom.read(consts.FLAT_MEM_FIELD) is not False
 
     def get_tx_power_support(self):
         return False

sonic_platform_base/sonic_xcvr/api/public/sff8472.py

@@ -261,7 +261,7 @@ def tx_disable_channel(self, channel, disable):
         return self.tx_disable(disable) if channel != 0 else True
 
     def is_flat_memory(self):
-        return not self.xcvr_eeprom.read(consts.PAGING_SUPPORT_FIELD)
+        return not self.xcvr_eeprom.read(consts.PAGING_SUPPORT_FIELD) is not False
 
     def get_temperature_support(self):
         return self.xcvr_eeprom.read(consts.DDM_SUPPORT_FIELD)

sonic_platform_base/sonic_xcvr/api/public/sff8636.py

@@ -299,7 +299,7 @@ def set_power_override(self, power_override, power_set):
         return ret
 
     def is_flat_memory(self):
-        return self.xcvr_eeprom.read(consts.FLAT_MEM_FIELD)
+        return self.xcvr_eeprom.read(consts.FLAT_MEM_FIELD) is not False
 
     def get_tx_power_support(self):
         if self.is_copper():

sonic_platform_base/sonic_xcvr/api/xcvr_api.py

@@ -493,7 +493,7 @@ def is_flat_memory(self):
         Returns:
             A Boolean, True if flat memory, False if paging is implemented
 
-            If there is an issue with reading the xcvr, None should be returned.
+            If there is an issue with reading the xcvr, True should be returned.
         """
         raise NotImplementedError