QSslCertificate: fromPath(): check the path arg isn't empty

`path` may contain wildcard glob patterns or regular expression ones,
so this method tries to get a path prefix without those special
characters, but that prefix could end up being empty, to counter
that, "." is used as a prefix. This led to iterating over the
current dir, which isn't the intended behavior. If the current dir
has many files, this could end up slowing things down (which is what
happens in the bug report).

Fix the issue by returning early if `path` is empty.

Amends 49f143e19ca11ef48260a3aaaa4ddbe490cf81ab.

[ChangeLog][QtNetwork][QSslCertificate][Important Behavior Changes]
fromPath() no longer accepts an empty path, which would previously
result in searching the current directory.

Fixes: QTBUG-134419
Pick-to: 6.8 6.5
Change-Id: I29224c3a47794b4095db5feae32a964dd9b854ff
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>
(cherry picked from commit 2875c4358beedd0997b60d21df4b95dbfec4f9a6)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/network/ssl/qsslcertificate.cpp

@@ -630,6 +630,9 @@ QList<QSslCertificate> QSslCertificate::fromPath(const QString &path,
                                                  QSsl::EncodingFormat format,
                                                  PatternSyntax syntax)
 {
+    if (path.isEmpty())
+        return {};
+
     // $, (,), *, +, ., ?, [, ,], ^, {, | and }.
 
     // make sure to use the same path separators on Windows and Unix like systems.