Don't pass null action to __doRequest

The parameter is not nullable, so it will be interpreted as
an empty string anyway.

The entire code here is pretty confusing though, and probably
deserves a second loop. The HTTP code only send SOAPAction/action
if soapaction is non-NULL -- but it always is, because it is
accepted through a non-nullable string parameter.

Regarding the SOAPAction header, it appears that always sending
it is actually a requirement of the standard:
> An HTTP client MUST use this header field when issuing a SOAP
> HTTP Request.
Although it does make a distinction between absence of value and
an empty string:
> The header field value of empty string ("") means that the intent
> of the SOAP message is provided by the HTTP Request-URI. No value
> means that there is no indication of the intent of the message.
The empty string interpretation appears to be the desired one.

However, for the action MIME tag the SOAP 1.2 Part 2 specification
says that
> The media type specifies an optional action parameter, which can
> be used to optimize dispatch or routing, among other things.
but also
> The SOAP Action feature defines a single property, which is
> described in Table 14. The value of this property MUST be an
> absolute URI[RFC 3986] and MUST NOT be empty.
which would indicate that we should not be sending an empty
action here.

As I'm not familiar with SOAP and this is long-standing behavior,
I'm just leaving this alone for now...

ext/soap/soap.c

@@ -2217,7 +2217,7 @@ static int do_request(zval *this_ptr, xmlDoc *request, char *location, char *act
 		ZVAL_STRINGL(&params[0], buf, buf_size);
 		ZVAL_STRING(&params[1], location);
 		if (action == NULL) {
-			ZVAL_NULL(&params[2]);
+			ZVAL_EMPTY_STRING(&params[2]);
 		} else {
 			ZVAL_STRING(&params[2], action);
 		}