QProcess: fix startCommand() with whitespace-only strings

We'd end up trying to takeFirst() from an empty QStringList.

[ChangeLog][QtCore][QProcess] Fixed a bug that would cause
startCommand() to crash if passed a string that was empty or contained
only whitespace characters.

Fixes: QTBUG-124512
Pick-to: 6.5 6.6 6.7
Change-Id: I455fe22ef4ad4b2f9b01fffd17c7689095c39272
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/corelib/io/qprocess.cpp

@@ -2281,6 +2281,10 @@ void QProcess::start(OpenMode mode)
 void QProcess::startCommand(const QString &command, OpenMode mode)
 {
     QStringList args = splitCommand(command);
+    if (args.isEmpty()) {
+        qWarning("QProcess::startCommand: empty or whitespace-only command was provided");
+        return;
+    }
     const QString program = args.takeFirst();
     start(program, args, mode);
 }

tests/auto/corelib/io/qprocess/tst_qprocess.cpp

@@ -48,6 +48,7 @@ private slots:
     void constructing();
     void simpleStart();
     void startCommand();
+    void startCommandEmptyString();
     void startWithOpen();
     void startWithOldOpen();
     void execute();
@@ -299,6 +300,25 @@ void tst_QProcess::startCommand()
     QCOMPARE(actual, expected);
 }
 
+void tst_QProcess::startCommandEmptyString()
+{
+    static const char warningMsg[] =
+            "QProcess::startCommand: empty or whitespace-only command was provided";
+    QProcess process;
+
+    QTest::ignoreMessage(QtWarningMsg, warningMsg);
+    process.startCommand("");
+    QVERIFY(!process.waitForStarted());
+
+    QTest::ignoreMessage(QtWarningMsg, warningMsg);
+    process.startCommand("   ");
+    QVERIFY(!process.waitForStarted());
+
+    QTest::ignoreMessage(QtWarningMsg, warningMsg);
+    process.startCommand("\t\n");
+    QVERIFY(!process.waitForStarted());
+}
+
 void tst_QProcess::startWithOpen()
 {
     QProcess p;