SqlSetup: Passes parameters with empty values which should not be passed for PrepareFailoverCluster

Details of the scenario you try and problem that is occurring:
Prepare fails with the following error:
(01) 2017-02-27 15:57:27 Slp: The credentials you provided for the SQL Server Agent service are invalid. To continue, provide a valid account and password for the SQL Server Agent service.
(01) 2017-02-27 15:57:27 Slp: HResult : 0x851c0001

Looking through the Summary log you see list of parms passed:
User Input Settings:
ACTION: PrepareFailoverCluster
AGTDOMAINGROUP:
AGTSVCACCOUNT:
AGTSVCPASSWORD:
ASPROVIDERMSOLAP: 1
ASSVCACCOUNT:
ASSVCPASSWORD:
ASSVCSTARTUPTYPE: Automatic
CONFIGURATIONFILE:
ENU: true
FEATURES: SQLENGINE, REPLICATION, FULLTEXT, DQ
FILESTREAMLEVEL: 0
FILESTREAMSHARENAME:
FTSVCACCOUNT: NT Service\MSSQLFDLauncher
FTSVCPASSWORD:
HELP: false
IACCEPTROPENLICENSETERMS: false
IACCEPTSQLSERVERLICENSETERMS: true
INDICATEPROGRESS: false
INSTALLSHAREDDIR: C:\Program Files\Microsoft SQL Server
INSTALLSHAREDWOWDIR: C:\Program Files (x86)\Microsoft SQL Server
INSTANCEDIR: C:\Program Files\Microsoft SQL Server
INSTANCEID: MSSQLSERVER
INSTANCENAME: MSSQLSERVER
ISSVCACCOUNT: NT AUTHORITY\Network Service
ISSVCPASSWORD:
ISSVCSTARTUPTYPE: Automatic
MRCACHEDIRECTORY:
PBDMSSVCACCOUNT:
PBDMSSVCPASSWORD:
PBDMSSVCSTARTUPTYPE: 0
PBENGSVCACCOUNT:
PBENGSVCPASSWORD:
PBENGSVCSTARTUPTYPE: 0
PBPORTRANGE:
PBSCALEOUT: false
PID: *****
QUIET: true
QUIETSIMPLE: false
RSINSTALLMODE: DefaultNativeMode
RSSHPINSTALLMODE: DefaultSharePointMode
RSSVCACCOUNT:
RSSVCPASSWORD:
RSSVCSTARTUPTYPE: Automatic
SQLSVCACCOUNT:
SQLSVCINSTANTFILEINIT: false
SQLSVCPASSWORD:
SUPPRESSPRIVACYSTATEMENTNOTICE: false
UIMODE: Normal
UpdateEnabled: true
UpdateSource: MU
USEMICROSOFTUPDATE: false
X86: false

For prepare most of these are not required.

The DSC configuration that is using the resource (as detailed as possible):
xSQLServerSetup "PrepareMSSQLSERVER"
{
DependsOn = @(
"[WindowsFeature]NET-Framework-Core",
"[WindowsFeature]Failover-Clustering"
)
Action = "PrepareFailoverCluster"
SourcePath = $Node.SourcePath
SetupCredential = $Node.InstallerServiceAccount
Features = $Node.Features
InstanceName = $Node.InstanceName
}
Version of the Operating System, SQL Server and PowerShell the DSC Target Node is running:
Windows 2016, SQL Server 2016, 5.1
Version of the DSC module you're using, or 'dev' if you're using current dev branch:
dev

Start at the xSQLServerSetup resource handling Action = "PrepareFailoverCluster" and inspect how setup parameters are assembled from the DSC configuration. The change is complete when optional parameters with empty values are omitted for prepare operations and the provided PrepareMSSQLSERVER configuration no longer passes those empty service credentials.