Avoid null coalescing operator for pwsh 6 support

Fixes #157417

src/vs/workbench/contrib/terminal/browser/media/shellIntegration.ps1

@@ -36,7 +36,11 @@ function Global:Prompt() {
 			# Sanitize the command line to ensure it can get transferred to the terminal and can be parsed
 			# correctly. This isn't entirely safe but good for most cases, it's important for the Pt parameter
 			# to only be composed of _printable_ characters as per the spec.
-			$CommandLine = $LastHistoryEntry.CommandLine ?? ""
+			if ($LastHistoryEntry.CommandLine) {
+				$CommandLine = $LastHistoryEntry.CommandLine
+			} else {
+				$CommandLine = ""
+			}
 			$Result += $CommandLine.Replace("`n", "<LF>").Replace(";", "<CL>")
 			$Result += "`a"
 			# Command finished exit code