cmake-gui: Fix regression that prints an incidental error on fresh build trees

Since commit a872844908 (cmake-gui: Handle relative paths in the build
directory text input, 2024-04-24, v3.30.0-rc1~5^2), `cmake-gui` gave an
incidental error message even when a valid build directory was being
passed.  Fix this by checking for a non-empty path before using it.

Fixes: #26744

Source/QtDialog/QCMake.cxx

@@ -94,7 +94,8 @@ void QCMake::setSourceDirectory(const QString& _dir)
     emit this->sourceDirChanged(this->SourceDirectory);
     this->loadPresets();
     this->setPreset(QString{});
-    if (!cmSystemTools::FileIsFullPath(
+    if (!this->MaybeRelativeBinaryDirectory.isEmpty() &&
+        !cmSystemTools::FileIsFullPath(
           this->MaybeRelativeBinaryDirectory.toStdString())) {
       this->setBinaryDirectory(this->MaybeRelativeBinaryDirectory);
     }