Another way to set metadata field value to null.You You should get TaxonomyField first.
TaxonomyFieldValue fieldValue = new TaxonomyFieldValue(field);
field.SetFieldValue(item,fieldValue);
Item.UpdateOverwriteVersion();
TaxonomyFieldValue fieldValue = new TaxonomyFieldValue(field);
field.SetFieldValue(item,fieldValue);
Item.UpdateOverwriteVersion();