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We thank G. Navarro for allowing us to include here his proof of Theorem 2.1. Research supported by Grant PID2019-103854GB-I00 funded by MCIN/AEI/10.13039/501100011033 and Generalitat Valenciana AICO/2020/298. The second author is also supported by “Convocatoria de contratación para la especialización de personal investigador doctor en la UPV/EHU (2019)” and Grant PID2020-118193GA-I00 funded by MCIN/AEI/10.13039/501100011033. Part of this work was done while the first author was visiting the University of the Basque Country. He thanks the Mathematics Department for its hospitality. Finally, we are obliged to the anonymous referees for accurate corrections on our article.