MONGOCRYPT-766 Fix compilation in C23

[jeroen]

kms/kms_request.c:792:14: warning: initialization of pointer of type
'char *' to null from a constant boolean expression
[-Wbool-conversion]
      return false;
             ^~~~~
kms/kms_request.c:860:14: warning: initialization of pointer of type
'char *' to null from a constant boolean expression
[-Wbool-conversion]
      return false;
             ^~~~~
kms/kms_request.c:864:14: warning: initialization of pointer of type
'char *' to null from a constant boolean expression
[-Wbool-conversion]
      return false;
             ^~~~~

[kevinAlbs]

Thank you! LGTM