Fix showing document prefix in document switcher

Retrieve the longest common prefix only for 2 or more paths, otherwise the prefix will be empty.

addons/tabswitcher/autotests/tabswitchertest.cpp

@@ -67,4 +67,11 @@ void KateTabSwitcherTest::testLongestCommonPrefix_data()
     strs.push_back(QStringLiteral("/home/autolink"));
     strs.push_back(QStringLiteral("/home/async"));
     QTest::newRow("find correct path prefix") << strs << QStringLiteral("/home/");
+
+    strs.clear();
+    QTest::newRow("empty list") << strs << QString();
+
+    strs.clear();
+    strs.push_back(QStringLiteral("/home/a"));
+    QTest::newRow("list with one item") << strs << QString();
 }

addons/tabswitcher/tabswitcherfilesmodel.cpp

@@ -40,18 +40,15 @@ QString FilenameListItem::fullPath() const
 }
 
 /**
- * Note that if strs contains the empty string, the result will be ""
+ * Note that if strs contains less than 2 items, the result will be an empty string.
  */
 QString longestCommonPrefix(std::vector<QString> const &strs)
 {
-    if (strs.empty()) {
+    // only 2 or more items can have a common prefix
+    if (strs.size() < 2) {
         return QString();
     }
 
-    if (strs.size() == 1) {
-        return strs.front();
-    }
-
     // get the min length
     auto it = std::min_element(strs.begin(), strs.end(), [](const QString &lhs, const QString &rhs) {
         return lhs.size() < rhs.size();