QCompleter: QCompleter unexpectedly changes QLineEdit text

When qcompleter and qlineedit are used together, the currentcompletion()
of qcompleter is its first item by default. Therefore, when qlineedit
makes the initial value, then selects the text and enters, qcompleter
will modify the default first item to qlineedit text. The judgment that
completionprefix() of the completer is not empty is modified and added
here, because completionprefix() is always empty when there is no match.

Fixes: QTBUG-20894
Change-Id: Id47f7f4da0a93d03a4e7b056ec731615b0803d13
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>
Reviewed-by: Tianlu Shao <shaotianlu@uniontech.com>
(cherry picked from commit 0ef190fcc429405200d9f6063851726fcfb75637)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/widgets/widgets/qwidgetlinecontrol.cpp

@@ -1658,6 +1658,7 @@ void QWidgetLineControl::processKeyEvent(QKeyEvent* event)
                     break;
 #endif
                 if (!m_completer->currentCompletion().isEmpty() && hasSelectedText()
+                    && !m_completer->completionPrefix().isEmpty()
                     && textAfterSelection().isEmpty()) {
                     setText(m_completer->currentCompletion());
                     inlineCompletionAccepted = true;