Remove default for curtailable flag in PV config (#84)

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

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."""