Fixes StringProperty not detecting changes to empty string properly

- sirius.db.mixing.properties.StringProperty#setValueToField and sirius.db.mixing.properties.StringProperty#transformValue
  transform "" be persisted as null
- this transformation must be considered when checking for null as well

Fixes: SIRI-939

src/main/java/sirius/db/mixing/properties/StringProperty.java

@@ -124,6 +124,14 @@ protected void setValueToField(Object value, Object target) {
         super.setValueToField(value, target);
     }
 
+    @Override
+    protected boolean isConsideredNull(Object propertyValue) {
+        if (trim) {
+            return Strings.isEmpty(Strings.trim(propertyValue));
+        }
+        return Strings.isEmpty(propertyValue);
+    }
+
     @Override
     public Object transformValue(Value value) {
         if (value.isFilled()) {

src/test/java/sirius/db/jdbc/DataTypesEntity.java

@@ -12,6 +12,7 @@
 import sirius.db.mixing.annotations.Length;
 import sirius.db.mixing.annotations.NullAllowed;
 import sirius.db.mixing.annotations.Numeric;
+import sirius.db.mixing.annotations.Trim;
 import sirius.kernel.commons.Amount;
 
 import java.time.LocalDate;
@@ -36,6 +37,11 @@ public enum TestEnum {
     @NullAllowed
     private String stringValue;
 
+    @Trim
+    @Length(255)
+    @NullAllowed
+    private String stringValueNoDefault;
+
     @DefaultValue("300")
     @NullAllowed
     @Numeric(precision = 20, scale = 3)
@@ -87,6 +93,14 @@ public void setStringValue(String stringValue) {
         this.stringValue = stringValue;
     }
 
+    public String getStringValueNoDefault() {
+        return stringValueNoDefault;
+    }
+
+    public void setStringValueNoDefault(String stringValueNoDefault) {
+        this.stringValueNoDefault = stringValueNoDefault;
+    }
+
     public Amount getAmountValue() {
         return amountValue;
     }

src/test/kotlin/sirius/db/jdbc/DataTypesTest.kt

@@ -16,6 +16,7 @@ import sirius.kernel.commons.Value
 import sirius.kernel.di.std.Part
 import java.time.Duration
 import kotlin.test.assertEquals
+import kotlin.test.assertFalse
 import kotlin.test.assertTrue
 
 @ExtendWith(SiriusExtension::class)
@@ -33,6 +34,18 @@ class DataTypesTest {
         assertEquals(Long.MAX_VALUE, test.longValue)
     }
 
+    @Test
+    fun `StringProperty detects changes to empty values properly`() {
+        var test = DataTypesEntity()
+        test.stringValueNoDefault = ""
+
+        oma.update(test)
+        val afterFirstSave = oma.refreshOrFail(test)
+
+        afterFirstSave.stringValueNoDefault = ""
+        assertFalse { afterFirstSave.isAnyMappingChanged }
+    }
+
     @Test
     fun `default values work when using parseValue()`() {
         var test = DataTypesEntity()