sendf: accept zero-length data in Curl_client_write()

Historically, Curl_client_write() used a length value of 0 as a marker
for a null-terminated data string. This feature has been removed in
commit f4b85d2. To detect leftover uses of the feature, a DEBUGASSERT
statement rejecting a length with value 0 was introduced, effectively
precluding use of this function with zero-length data.

The current commit removes the DEBUGASSERT and makes the function to
return immediately if length is 0.

A direct effect is to fix trying to output a zero-length distinguished
name in openldap.

Another DEBUGASSERT statement is also rephrased for better readability.

Closes #7898

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