iOS: Fix determination of SDK with maximum version

Change-Id: I6fc1f0eceeff4286936999986263cb034c1a7663
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@digia.com>

src/plugins/ios/iosprobe.cpp

@@ -248,7 +248,7 @@ void IosProbe::setupDefaultToolchains(const QString &devPath, const QString &xco
                         continue;
                     }
                     if (sdkName.isEmpty()) {
-                        if (compareVersions(maxVersion, versionStr) > 0) {
+                        if (maxVersion.isEmpty() || compareVersions(maxVersion, versionStr) > 0) {
                             maxVersion = versionStr;
                             sdkPath = sdkDirInfo.canonicalFilePath();
                             sdkSettings = sdkInfo;