Fix no-clause case-lambda in peval

This is checked by optargs.test, which 3b47f87618
broke.

* module/language/tree-il/peval.scm (peval): Account for no clause (= no body)
  case-lambda.

doc/ref/api-control.texi

@@ -1323,7 +1323,7 @@ exception handlers are invoked within the continuation of the error,
 without unwinding the stack.  The dynamic environment of the handler
 call will be that of the @code{raise-exception} call, with the
 difference that the current exception handler will be ``unwound'' to the
-\"outer\" handler (the one that was in place when the corresponding
+``outer'' handler (the one that was in place when the corresponding
 @code{with-exception-handler} was called).
 
 However, it's often the case that one would like to handle an exception

module/language/tree-il/peval.scm

@@ -1671,6 +1671,7 @@ top-level bindings from ENV and return the resulting expression."
            (($ <lambda> src-proc meta orig-body)
             ;; If there are multiple cases and one matches nargs, omit all the others.
             (or (and
+                 orig-body
                  (lambda-case-alternate orig-body)
                  (let ((nargs (length orig-args)))
                    (let loop ((body orig-body))