[partition] Fix up tests

- Although we long ago replaced the getPartitions implementation, the
  test is still there, and on a machine with no /dev/sda (e.g. because
  root is on nvme) the echo-awk-shell-pipeline can give an empty string;
  this is turned into a QStringList{""} which has one element, while
  the new version has 0 elements.
- Special-case the test that empty strings should be empty lists, rather
  than 1-element lists with an empty element.

src/modules/partition/tests/ClearMountsJobTests.cpp

@@ -42,7 +42,11 @@ getPartitionsForDevice_other(const QString& deviceName)
     process.start();
     process.waitForFinished();
 
-    const QString partitions = process.readAllStandardOutput();
+    const QString partitions = process.readAllStandardOutput().trimmed();
+    if ( partitions.isEmpty() )
+    {
+        return QStringList();
+    }
     const QStringList partitionsList = partitions.simplified().split( ' ' );
 
     return partitionsList;