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from my perspective "lack-of-value" is properly encoded in SLD, it is however not correctly interpreted by the SLD importer

<se:Rule> 
          <se:Name>Achterhoek</se:Name>
          <se:Description>
            <se:Title>Achterhoek</se:Title>
          </se:Description>
          <ogc:Filter xmlns:ogc="http://www.opengis.net/ogc">
            <ogc:PropertyIsEqualTo>
              <ogc:PropertyName>omgdnst_nm</ogc:PropertyName>
              <ogc:Literal>Achterhoek</ogc:Literal>
            </ogc:PropertyIsEqualTo>
          </ogc:Filter>
...
<se:Rule> <!-- all features get this style in qgis, seems this rule is interpreted as "always applies", and since it is last, it overwrites previous styles -->
          <se:Name></se:Name>
          <se:Description>
            <se:Title>omgdnst_nm is ''</se:Title>
          </se:Description>
          <ogc:Filter xmlns:ogc="http://www.opengis.net/ogc">
            <ogc:PropertyIsEqualTo>
              <ogc:PropertyName>omgdnst_nm</ogc:PropertyName>
              <ogc:Literal></ogc:Literal>
            </ogc:PropertyIsEqualTo>
          </ogc:Filter>

this last rule is interpreted as <rule label="omgdnst_nm is ''" key="{5a36ca93-4ba6-4b1e-a0b5-e2ad19fb4387}" symbol="7"/>

in stead it should be interpreted as

<rule filter="omgdnst_nm = ''" label="omgdnst_nm is ''" key="{5a36ca93-4ba6-4b1e-a0b5-e2ad19fb4387}" symbol="7"/>

however this gives the problem that features with value '' are not displayed anymore in qgis, this is what andrea speaks about in #16706

Curiously this filter works: concat(omgdnst_nm,'dummy')='dummy'

1) Is it possible to create/add a little dataset+sld's to this issue? So we all talk about the same things/rules?

2) Note that Andrea Aime has done a lot of sld related work in current master (upcoming 3.0), which was very hard to backport to 2.18.
So I would recommend to check if this is still the case in current master, and then fix (only) there.

3) Note that for what I know QGIS is always morphing it's own categorized and graduated styling to rulebased styling when EXPORTING to sld.
When IMPORTING sld, it always becomes rule based symbology if I am right?

Some questions I am curious about are:

- if several rules apply (like a match and a 'catch all' rule), should QGIS (and also in the sld world) then apply both rules, or only the first one matching? Because in the second case, ORDER of the rules becomes very important.

- is handling quotes, emtpy values and null values the same between QGIS and sld/geoserver?

Anton Bakker wrote:

I found that when the "other" filter class is listed first in class list this class is not overwriting the other classes, but is when this class is last in the class list. So when loading an SLD, when the rule of a filter class is empty it should be inserted at the top of the classes list.

The same issue persist on QGIS 3.0 (running on windows 10)

Please test with QGIS 3.4 - QGIS 2.18 reached it's end of life.