Fix increase by percentage returns 1 if experience is equal to zero (#8425)

Using `[effect]app/y_to,increase=experience,"40%"` on a unit with 0 XP used
to give that unit 1 XP, because of a minimum in the C++.

data/test/scenarios/wml_tests/UnitsWML/increase_experience_by_percentage.cfg

@@ -0,0 +1,44 @@
+# wmllint: no translatables
+
+#####
+# API(s) being tested: [effect]apply_to=experience
+##
+# Actions:
+# Try to set Alice's and Bob's experience to 0 and increase by 40%.
+##
+# Expected end state:
+# Alice and Bob have 0 experience.
+#####
+{COMMON_KEEP_A_B_UNIT_TEST "increase_zero_experience_by_percentage" (
+    [event]
+        name=start
+        [modify_unit]
+            [filter]
+                id=alice,bob
+            [/filter]
+            [effect]
+                apply_to=experience
+                set=0
+            [/effect]
+            [effect]
+                apply_to=experience
+                increase=40%
+            [/effect]
+        [/modify_unit]
+        [store_unit]
+            [filter]
+                id=alice
+            [/filter]
+            variable=a
+        [/store_unit]
+        [store_unit]
+            [filter]
+                id=bob
+            [/filter]
+            variable=b
+        [/store_unit]
+        {ASSERT ({VARIABLE_CONDITIONAL a.experience numerical_equals 0})}
+        {ASSERT ({VARIABLE_CONDITIONAL b.experience numerical_equals 0})}
+        {SUCCEED}
+    [/event]
+)}

src/units/unit.cpp

@@ -2214,7 +2214,7 @@ void unit::apply_builtin_effect(std::string apply_to, const config& effect)
 		}
 
 		if(increase.empty() == false) {
-			experience_ = utils::apply_modifier(experience_, increase, 1);
+			experience_ = utils::apply_modifier(experience_, increase, 0);
 		}
 	} else if(apply_to == "max_experience") {
 		const std::string& increase = effect["increase"];

wml_test_schedule

@@ -495,6 +495,7 @@
 0 kill_fires_events
 # Game mechanics
 0 zero_experience
+0 increase_zero_experience_by_percentage
 0 unit_no_advancements
 0 no_sides
 # heal ability tests