Fix Load Data from CSV File for Value Map widget

[agiudiceandrea]

Description

• set UTF-8 codec when reading the CSV file

• correctly handle CSV file with a single column without a trailing comma, like

1
2
3

while currently only a CSV like the following one is allowed

1,
2,
3,

Fixes #60765.

(proposed again after #61568 and #62591 were closed by the stale bot)

P.S. @nyalldawson, it seems to me using QString( "" ) (instead of QString() as you proposed) is needed: it is due to the fact that QgsValueMapConfigDlg::config() behaves differently if the QTableWidgetItem.text() of the value is a QString()

QGIS/src/gui/editorwidgets/qgsvaluemapconfigdlg.cpp

Lines 72 to 79 in 69a79ed

	if ( !vi || vi->text().isNull() )
	{
	value.insert( ks, ks );
	}
	else
	{
	value.insert( vi->text(), ks );
	}

When a row like 1, is loaded, the regex has 2 matches: 1 for the key and an empty (not null) QString for the value. The output of QgsValueMapConfigDlg::config() will contain a map with the key 1 and an empty (not null) QString as the value.

When a row like 1 is loaded, the regex has 1 match: 1 for the key.
If we use a null QString for the value, then the output of QgsValueMapConfigDlg::config() will contain a map with the key 1 and also 1 as the value, instead of a map with the key 1 and an empty QString as the value.
We need to use an empty (not null) QString as the value, in order to make QgsValueMapConfigDlg::config() output the same map like when a row like 1, is loaded.

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[nyalldawson]

When a row like 1, is loaded, the regex has 2 matches: 1 for the key and an empty (not null) QString for the value. The output of QgsValueMapConfigDlg::config() will contain a map with the key 1 and an empty (not null) QString as the value.

When a row like 1 is loaded, the regex has 1 match: 1 for the key.
If we use a null QString for the value, then the output of QgsValueMapConfigDlg::config() will contain a map with the key 1 and also 1 as the value, instead of a map with the key 1 and an empty QString as the value.
We need to use an empty (not null) QString as the value, in order to make QgsValueMapConfigDlg::config() output the same map like when a row like 1, is loaded.

This is very messy, and it's part of the reason why null QString logic is discouraged. From the qt docs: https://doc.qt.io/qt-6/qstring.html#distinction-between-null-and-empty-strings "We recommend that you always use the isEmpty() function and avoid isNull()."

Can the dialog be reworked to avoid the null strings entirely?