Add blank string handling for CustomValue::DateStrategy

Other strategies like BoolStrategy and LinkStrategy have a similar
parse_value override to handle blank strings. This will equate '' to nil
for the filter and thus fix the bug (OP-19456)

app/models/custom_value/date_strategy.rb

@@ -31,6 +31,10 @@
 class CustomValue::DateStrategy < CustomValue::FormatStrategy
   include Redmine::I18n
 
+  def parse_value(val)
+    val.presence
+  end
+
   def typed_value
     return if value.blank?
 

spec/models/custom_value/date_strategy_spec.rb

@@ -133,4 +133,27 @@ RSpec.describe CustomValue::DateStrategy do
       end
     end
   end
+
+  describe "#parse_value" do
+    subject { instance.parse_value(val) }
+    let(:value) { nil } # unused by parse_value but required by the double
+
+    context "when value is a valid date string" do
+      let(:val) { "2025-07-03" }
+
+      it { is_expected.to eq "2025-07-03" }
+    end
+
+    context "when value is an empty string" do
+      let(:val) { "" }
+
+      it { is_expected.to be_nil }
+    end
+
+    context "when value is nil" do
+      let(:val) { nil }
+
+      it { is_expected.to be_nil }
+    end
+  end
 end