Presumably, all numeric values shall be written as i4

...in 47c5454ea67632278d69a1ddfe97d74f5cc4449f "fdo#83428: Custom Properties
dropped while exporting to docx," not just ones of UNO type double.  At least,
that fixes CppunitTest_sc_subsequent_export_test's
testPivotTableTwoDataFieldsXLSX to not write a "DocSecurity" <property> without
content (and thus fail validation).

Change-Id: I2d764e65ec2af61139c6564f644d27aa9a00bd7d