Fix bug that made preprocessor with fx files fail in rare cases

[schlunma]

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Closes #448.

[schlunma]

Circle CI fails because the following test:

ESMValCore/tests/integration/cmor/test_table.py

Lines 292 to 301 in cac2d43

	def test_get_variable_from_custom(self):
	"""Get a variable from default."""
	self.variables_info.strict = False
	var = self.variables_info.get_variable('A1', 'swcre')
	self.assertEqual(var.short_name, 'swcre')
	self.assertEqual(var.frequency, '')
	var = self.variables_info.get_variable('day', 'swcre')
	self.assertEqual(var.short_name, 'swcre')
	self.assertEqual(var.frequency, '')

This fails because var.frequency is None in the second case now instead of the expected ''. Is this behavior intended @jvegasbsc ? In this test, the custom table swcre is read, which does not specify frequency. So I guess somehow the use of day sets the frequency to None. Before this fix, the frequency was (falsely) set to '' in the call var = self.variables_info.get_variable('A1', 'swcre'), so this test didn't fail.

All other tables entries are fine though (e.g. long_name, dimensions, etc.), only the frequency part fails.

[jvegreg]

The frequency for custom variables was set to None at read time. I changed it to be set to the empty string so it is consistent with all the others

[schlunma]

I successfully tested recipe_anav13jclim.yml with this, so this fixes ESMValGroup/ESMValTool#1548.

[bouweandela]

@mattiarighi Could you please run a final test and merge?

[valeriupredoi]

cheers @schlunma 🍺