Update CodingStyle to reflect the fact that 'nullptr' is supported also in C code in freeciv-3.3
While I'd like even recommend it over 'NULL', more importantly 'nullptr' should not leak (in backports) to freeciv-3.2, wording CodingStyle so it would not push people to take unnecessary risks (with code that might get backported later).
Update CodingStyle to reflect the fact that 'nullptr' is supported also in C code in freeciv-3.3
While I'd like even recommend it over 'NULL', more importantly 'nullptr' should not leak (in backports) to freeciv-3.2, wording CodingStyle so it would not push people to take unnecessary risks (with code that might get backported later).