Fix Le_Keylength checks during renewals

Fix Le_Keylength checks during renewals

When performing renewals acme.sh checks key length values to determine
if a new key should be created with createDomainKey(). However, older
acme.sh stored key length as an empty value if the default of 2048 was
desired. Now it is explicit and the explict check of 2048 against "" is
causing createDomainKey() to always be called with fails without
--force.

Fix this by converting the keylength value to 2048 if an empty string is
returned from the config file. acme.sh will then write out 2048 updating
old keys and configs to the explicit version.

Issue: 4077