empty XML field causes PHP Catchable fatal error

[justindossey]

Observed behavior:
with a billTo block in an XML request like this:

  <billTo>
    <firstName>Friday</firstName>
    <lastName>Test</lastName>
    <street1>123 Test St</street1>
    <street2></street2>
    <city>My City</city>
    <state>CA</state>
    <postalCode>12345</postalCode>
    <country>US</country>
    <email>nobody@cybersource.com</email>
    <customerID>56</customerID>
  </billTo>

I get
PHP Catchable fatal error: Object of class stdClass could not be converted to string in lib/CybsSoapClient.php on line 89
This is because the street2 field has no data. Removing that field causes the request to succeed.

Expected behavior:
the request will succeed even with an empty field.