Fix CGMES VoltageLevel high/low voltage warnings (#3831)

* Fix CGMES VoltageLevel high/low voltage warnings
* clean

Signed-off-by: Damien Jeandemange <damien.jeandemange@artelys.com>

Author: jeandemanged

cgmes/cgmes-conversion/src/main/java/com/powsybl/cgmes/conversion/elements/VoltageLevelConversion.java

@@ -76,14 +76,23 @@ public void convert() {
             addAliases(vl);
 
             // Check voltage limit consistency.
-            if (lowVoltageLimit <= highVoltageLimit) {
-                vl.setHighVoltageLimit(highVoltageLimit);
-                vl.setLowVoltageLimit(lowVoltageLimit);
-                vl.setProperty(PROPERTY_HIGH_VOLTAGE_LIMIT, String.valueOf(highVoltageLimit));
-                vl.setProperty(PROPERTY_LOW_VOLTAGE_LIMIT, String.valueOf(lowVoltageLimit));
-            } else {
+            if (highVoltageLimit < lowVoltageLimit) {
+                // this code path is never reached if either of the values is NaN, i.e. not provided in input
                 ignored("high/low voltage limits",
-                    () -> String.format("lowVoltageLimit (%f) is greater than highVoltageLimit (%f).", lowVoltageLimit, highVoltageLimit));
+                        () -> String.format("lowVoltageLimit (%f) is greater than highVoltageLimit (%f).", lowVoltageLimit, highVoltageLimit));
+            } else {
+                // either:
+                // - no limits defined
+                // - or, only one limit is defined
+                // - or, both limits are defined and they are consistent
+                if (!Double.isNaN(highVoltageLimit)) {
+                    vl.setHighVoltageLimit(highVoltageLimit);
+                    vl.setProperty(PROPERTY_HIGH_VOLTAGE_LIMIT, String.valueOf(highVoltageLimit));
+                }
+                if (!Double.isNaN(lowVoltageLimit)) {
+                    vl.setLowVoltageLimit(lowVoltageLimit);
+                    vl.setProperty(PROPERTY_LOW_VOLTAGE_LIMIT, String.valueOf(lowVoltageLimit));
+                }
             }
         }
     }