file:read-text: allow empty sequence for option encoding

[GuntherRademacher]

There were two QT4 test failures:

EXPath-file-read-fallback-003
file:read-text("non-xml.txt", { "encoding": (), "fallback": true() })
Error : XPTY0004: Option encoding: xs:string expected, empty-sequence() found: ().
Expect: <assert xmlns="http://www.w3.org/2010/09/qt-fots-catalog">starts-with($result, "U0000=�")</assert>

EXPath-file-read-text-lines-fallback-003
file:read-text-lines("non-xml.txt", { "encoding": (), "fallback": true() })
Error : XPTY0004: Option encoding: xs:string expected, empty-sequence() found: ().
Expect: <assert xmlns="http://www.w3.org/2010/09/qt-fots-catalog">starts-with($result, "U0000=�")</assert>

These were caused by recent changes of those tests, introducing usage of the options parameter. This uncovered the option value not allowing emtpy sequence as a value, which is permitted by the spec.

[ChristianGruen]

Related: d6c0921