GenericLinuxDeviceTester: Use FileTransfer for sftp and rsync

Make ProcessArgs::prepareCommand() return false in case
when executable is a relative path and it can't be found.

Amends 13067a7d9c

Change-Id: I0c4cf1580be34894df01245eb2411cdd7a122651
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>

src/libs/utils/commandline.cpp

@@ -658,6 +658,8 @@ bool ProcessArgs::prepareCommand(const CommandLine &cmdLine, QString *outCmd, Pr
     const QString arguments = cmdLine.arguments();
     if (env && executable.isRelativePath())
         executable = env->searchInPath(executable.toString());
+    if (executable.isEmpty())
+        return false;
     ProcessArgs::SplitError err;
     *outArgs = ProcessArgs::prepareArgs(arguments, &err, executable.osType(), env, pwd);
     if (err == ProcessArgs::SplitOk) {