REST receiver: URL placeholder ... is not configured, or has an empty value

I have configured a scenario with a REST receiver for consuming a REST Service. My URL looks like that:

http://server:port/service?para1={para1}&para2={para2}

The para1 and para2 are in my request payload. The problem is that these parameters are optional. It is possible that they have no value. And the REST service allows calls like this:

http://server:port/service?para1=&para2=value2

But I get always an error when I try to call the service via the REST receiver with an empty parameter.

"MP: exception caught with cause com.sap.aii.adapter.rest.ejb.receiver.PlaceholderMissingException: URL placeholder para1 is not configured, or has an empty value"

In my real scenario I have more than 2 parameters and one or more of them are always empty. Is there a solution for this problem? Otherwise I can't use this for my purpose or I have to find a workaround (combine all parameter in one)