Add ability to leave SourceBranch empty when queueing a build (builds using the default branch) (#2691)

Co-authored-by: Daniel Jurek <djurek@microsoft.com>

eng/common/scripts/Invoke-DevOpsAPI.ps1

@@ -13,7 +13,6 @@ function Start-DevOpsBuild {
   param (
     $Organization="azure-sdk",
     $Project="internal",
-    [Parameter(Mandatory = $true)]
     $SourceBranch,
     [Parameter(Mandatory = $true)]
     $DefinitionId,

eng/common/scripts/Queue-Pipeline.ps1

@@ -1,3 +1,45 @@
+<#
+.SYNOPSIS
+Queues an Azure DevOps Pipeline run optionally canceling similar runs
+
+.PARAMETER Organization
+Azure DevOps organization name
+
+.PARAMETER Project
+Azure DevOps project name
+
+.PARAMETER SourceBranch
+Source branch use when executing the DevOps pipeline. Specifying an empty string
+will result in queuing of the run with the default branch configured for the
+pipeline.
+
+.PARAMETER DefinitionId
+Pipline definition ID
+
+.PARAMETER CancelPreviousBuilds
+Requires a value for SourceBranch. Cancel previous builds before queuing the new 
+build.
+
+.PARAMETER VsoQueuedPipelines
+Variable name to set in DevOps for the queued pipeline links
+
+.PARAMETER Base64EncodedAuthToken
+Auth token for Azure DevOps API
+
+.PARAMETER BuildParametersJson
+Additional build parameters to provide to the pipeline execution.
+
+Of the format:
+
+```json
+{
+  "variable1": "value1",
+  "variable2": "value2"
+}
+```
+
+#>
+
 [CmdletBinding(SupportsShouldProcess = $true)]
 param(
   [Parameter(Mandatory = $true)]
@@ -6,7 +48,6 @@ param(
   [Parameter(Mandatory = $true)]
   [string]$Project,
 
-  [Parameter(Mandatory = $true)]
   [string]$SourceBranch,
 
   [Parameter(Mandatory = $true)]
@@ -26,7 +67,9 @@ param(
 
 . (Join-Path $PSScriptRoot common.ps1)
 
-if ($CancelPreviousBuilds)
+# Skip if SourceBranch is empty because it we cannot generate a target branch
+# name from an empty string.
+if ($CancelPreviousBuilds -and $SourceBranch)
 {
   try {
     $queuedBuilds = Get-DevOpsBuilds -BranchName "refs/heads/$SourceBranch" -Definitions $DefinitionId `