PluginCertInfo can generate bad XML, causing crash

In some circumstances, `_create_xml_node` in `PluginCertInfo.py` gets called with a key like "`<Foo>`".  When the resulting xml tree is serialized and re-parsed with minidom in `sslyze.py`, the application crashes due to the invalid XML (`<<Foo> />`).

One example is the cert below, where the subjectAltName value is an empty string, resulting in a node named `<ParseError>`.  I've also seen `<Empty>` in the same field for another cert.

This commit is a quick tweak to prevent the application from crashing; I'm not sure if there's a better way to handle bogus values in certs in general, either within sslyze or down in nassl.

    -----BEGIN CERTIFICATE-----
    MIICrDCCAZSgAwIBAgIJANyuko6C2HhwMA0GCSqGSIb3DQEBCwUAMA8xDTALBgNV
    BAMMBGRlbW8wHhcNMTQxMjExMjAwNTQ2WhcNMjQxMjA4MjAwNTQ2WjAPMQ0wCwYD
    VQQDDARkZW1vMIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA11ooS5OM
    rUo1Viak3hhE9eIR1tPuFanO2+jowLAImytPiWxuBlVFINW+EX7TNEZJcrf52MtA
    Opy19unT7c0dAjVh7pIxjCgMvH0yecVdt3DHUg3ItyMBJWTI1t+t2+IQMaHCLOsS
    Ky29lgk8pegYR0A2O0D6SayjI0lXS2GW78BAnJJIhmluSrBLKHLrwNQZ2hWTrM9H
    VgRWDI49r9nI7sM7aXUdabTKQmJlsgUbigXEB2Z/myM+RN+zBTfbnSa9V5Asn+gR
    5YGQ1jIEOuuIJzd/+hHyO4xUPkki0n3ECMLyFOo6bMi0uSKfxsDIckzloF2TRuk/
    DFzIDQ5mz/MEdQIDAQABowswCTAHBgNVHREEADANBgkqhkiG9w0BAQsFAAOCAQEA
    OAgMKke3Ha6BL47eEzsGsnD1GC1Bjr6wC7tlTWfn1ykadcn9WnXIUOTAERFPtSnB
    oKvoKBWhX6yw1CKLdg4S4bFZSDzlKPNnmEnGqlKKQL1I3/TNCIZ8gxAtOi1KOjiW
    3JQCP0tbVMRTClCsRlP22/8esvO0lYAeamOpOjgSWiNmpToVKh7+Pcj2MpIcUJMe
    vSk+NtL9Mcgw6sM0Fdm/Vh3Xr2LaAk+oufA5eBC9nlq/C+Gm3K6Yiy2BRVfBvxe+
    wC54Fu3fdcD2ITymImBgYq2Qje9Ct3WZDGu/zYc6vxCgboHJFIZfE1GRFLmWQNhR
    Y6B3d9EgNuv4ve87itHIMQ==
    -----END CERTIFICATE-----