Don't crash in wxTextFile::GetLastLine() if the file is empty

Just return a reference to an empty wxString, as GetFirstLine() already does
(although this is actually questionable -- what if this string is modified?).

See #17283.

tests/textfile/textfiletest.cpp

@@ -117,6 +117,9 @@ void TextFileTestCase::ReadEmpty()
     CPPUNIT_ASSERT( f.Open(wxString::FromAscii(GetTestFileName())) );
 
     CPPUNIT_ASSERT_EQUAL( (size_t)0, f.GetLineCount() );
+    CPPUNIT_ASSERT( f.Eof() );
+    CPPUNIT_ASSERT_EQUAL( "", f.GetFirstLine() );
+    CPPUNIT_ASSERT_EQUAL( "", f.GetLastLine() );
 }
 
 void TextFileTestCase::ReadDOS()