Fix: Allow value attribute in height_unit element

Same fix as height element - supports attribute-only format:
<height_unit value='0'/> (ATAK format)

Was rejecting empty text content when only attribute present.

Author: pdfinn

validator/schemas/details/height_unit.xsd

@@ -1,4 +1,11 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified">
-  <xs:element name="height_unit" type="xs:integer"/>
+  <xs:complexType name="detail_height_unit">
+    <xs:simpleContent>
+      <xs:extension base="xs:string">
+        <xs:attribute name="value" type="xs:integer"/>
+      </xs:extension>
+    </xs:simpleContent>
+  </xs:complexType>
+  <xs:element name="height_unit" type="detail_height_unit"/>
 </xs:schema>