Remove default for curtailable flag in PV config

Allows to detect if the flag was not set by the user and let downstream
decide how to handle the unset case.

Signed-off-by: cwasicki <126617870+cwasicki@users.noreply.github.com>

Author: cwasicki

RELEASE_NOTES.md

@@ -6,7 +6,7 @@
 
 ## Upgrading
 
-<!-- Here goes notes on how to upgrade from previous versions, including deprecations and what they should be replaced with -->
+* Change the `curtailable` flag default of the PV config to `None`.
 
 ## New Features
 

src/frequenz/gridpool/_microgrid_config.py

@@ -117,7 +117,7 @@ class PVConfig:
     rated_power: float | None = None
     """Rated power of the inverters in Watt."""
 
-    curtailable: bool = False
+    curtailable: bool | None = None
     """Flag to indicate if PV system can be curtailed."""