[ test show platform ] skip firmware status if not implemented (#5068)

What is the motivation for this PR?
To avoid TC fail in case command returns empty output

How did you do it?
Add skip mark

How did you verify/test it?
Run TC test_verify_show_platform_firmware_status_output , TC skipped

Any platform specific information?
Will be skipped for platforms when test_verify_show_platform_firmware_status_output is not implemented

tests/platform_tests/cli/test_show_platform.py

@@ -344,6 +344,9 @@ def verify_show_platform_firmware_status_output(raw_output_lines, hostname):
     """
     NUM_EXPECTED_COLS = 5
 
+    # Skip if command not implemented for platform
+    if len(raw_output_lines) <= 2:
+        pytest.skip("show platform firmware status not implemented")
     pytest_assert(len(raw_output_lines) > 2, "There must be at least two lines of output on '{}'".format(hostname))
     second_line = raw_output_lines[1]
     field_ranges = util.get_field_range(second_line)