ENH: Values can hold empty lists now.

doc/Install_develop.rst

@@ -189,11 +189,14 @@ For the tests there is a slightly different setup required (with option
     - Depending on the clang version it may be necessary to also add
       ``-DCMAKE_CXX_FLAGS="-Wno-unused-parameter"``
 4. ``cmake --build .``
+    - If this fails with ``Error running '': No such file or directory``, you may want to try
+      CMake's ``-D SKIP_LINTING=ON``. (See previous step.)
 
 Run
 ^^^
 
-In the build directory, run ``ctest``
+In the build directory, run ``ctest``.  For more verbose output of a single test:
+``ctest -R test_value.test_list -V``
 
 Framework
 ^^^^^^^^^

include/caosdb/value.h

@@ -36,6 +36,10 @@
     for (const auto &value : values) {                                                             \
       this->wrapped->mutable_list_values()->add_values()->SETTER(value);                           \
     }                                                                                              \
+    if (values.empty()) {                                                                          \
+      this->wrapped->mutable_list_values()->add_values()->set_special_value                        \
+        (ProtoSpecialValue::SPECIAL_VALUE_UNSPECIFIED);                                            \
+    }                                                                                              \
   }
 
 namespace caosdb::entity {
@@ -385,7 +389,9 @@ public:
     return !IsNull() && this->wrapped->value_case() == ValueCase::kListValues;
   }
   [[nodiscard]] inline auto GetAsVector() const noexcept -> const std::vector<ScalarValue> & {
-    if (!IsVector()) {
+    if (!IsVector() || (this->wrapped->list_values().values(0).has_special_value()
+                        && this->wrapped->list_values().values(0).special_value()
+                        == ProtoSpecialValue::SPECIAL_VALUE_UNSPECIFIED)) {
       // create empty list
       static std::vector<ScalarValue> empty_values;
       return empty_values;