Fix push script passing in optional argument (Azure#19594)

With new PS command line parsing update in 7.3 they explicitly
pass "" to the native commands which in our usage here we don't
want. However setting the variable to null instead works for our
scenario with both command line parsing strategies so using it here.

Co-authored-by: Wes Haggard <Wes.Haggard@microsoft.com>

Author: azure-sdk

eng/common/scripts/git-branch-push.ps1

@@ -52,7 +52,7 @@ if ((git remote) -contains $RemoteName)
     exit 1
   }
 }
-else 
+else
 {
   Write-Host "git remote add $RemoteName $GitUrl"
   git remote add $RemoteName $GitUrl
@@ -66,8 +66,8 @@ else
 git show-ref --verify --quiet refs/heads/$PRBranchName
 if ($LASTEXITCODE -eq 0) {
   Write-Host "git checkout $PRBranchName."
-  git checkout $PRBranchName 
-} 
+  git checkout $PRBranchName
+}
 else {
   Write-Host "git checkout -b $PRBranchName."
   git checkout -b $PRBranchName
@@ -83,10 +83,11 @@ if (!$SkipCommit) {
         $amendOption = "--amend"
     }
     else {
-        $amendOption = ""
+        # Explicitly set this to null so that PS command line parser doesn't try to parse pass it as ""
+        $amendOption = $null
     }
-    Write-Host "git -c user.name=`"azure-sdk`" -c user.email=`"azuresdk@microsoft.com`" commit $amendOption -am `"$($CommitMsg)`""
-    git -c user.name="azure-sdk" -c user.email="azuresdk@microsoft.com" commit $amendOption -am "$($CommitMsg)"
+    Write-Host "git -c user.name=`"azure-sdk`" -c user.email=`"azuresdk@microsoft.com`" commit $amendOption -am `"$CommitMsg`""
+    git -c user.name="azure-sdk" -c user.email="azuresdk@microsoft.com" commit $amendOption -am "$CommitMsg"
     if ($LASTEXITCODE -ne 0)
     {
         Write-Error "Unable to add files and create commit LASTEXITCODE=$($LASTEXITCODE), see command output above."
@@ -115,7 +116,7 @@ do
     {
         $needsRetry = $true
         Write-Host "Git push failed with LASTEXITCODE=$($LASTEXITCODE) Need to fetch and rebase: attempt number=$($tryNumber)"
- 
+
         Write-Host "git fetch $RemoteName $PRBranchName"
         # Full fetch will fail when the repo is in a sparse-checkout state, and single branch fetch is faster anyway.
         git fetch $RemoteName $PRBranchName
@@ -162,8 +163,8 @@ do
                 continue
             }
 
-            Write-Host "git -c user.name=`"azure-sdk`" -c user.email=`"azuresdk@microsoft.com`" commit -m `"$($CommitMsg)`""
-            git -c user.name="azure-sdk" -c user.email="azuresdk@microsoft.com" commit -m "$($CommitMsg)"
+            Write-Host "git -c user.name=`"azure-sdk`" -c user.email=`"azuresdk@microsoft.com`" commit -m `"$CommitMsg`""
+            git -c user.name="azure-sdk" -c user.email="azuresdk@microsoft.com" commit -m "$CommitMsg"
             if ($LASTEXITCODE -ne 0)
             {
                 Write-Error "Unable to commit LASTEXITCODE=$($LASTEXITCODE), see command output above."
@@ -183,7 +184,7 @@ do
 if ($LASTEXITCODE -ne 0 -or $tryNumber -gt $numberOfRetries)
 {
     Write-Error "Unable to push commit after $($tryNumber) retries LASTEXITCODE=$($LASTEXITCODE), see command output above."
-    if (0 -eq $LASTEXITCODE) 
+    if (0 -eq $LASTEXITCODE)
     {
         exit 1
     }