cmake/interpreter: Remove None from values we promise wont have None

We've documented these lists as being `List[Path]`, but then we have the
potential to insert a None into them via the `rel_path()` function,
which can return `None` in some cases. We need to actually remove them
from the list before we assign, so that it's actually a `List[Path]`.
While we're here I've simplified the logic a bit.

Closes: mesonbuild#13551

mesonbuild/cmake/interpreter.py

@@ -8,6 +8,7 @@
 from functools import lru_cache
 from os import environ
 from pathlib import Path
+import itertools
 import re
 import typing as T
 
@@ -416,11 +417,14 @@ def rel_path(x: Path, is_header: bool, is_generated: bool) -> T.Optional[Path]:
                 return x.relative_to(root_src_dir)
             return x
 
+        def non_optional(inputs: T.Iterable[T.Optional[Path]]) -> T.List[Path]:
+            return [p for p in inputs if p is not None]
+
         build_dir_rel = self.build_dir.relative_to(Path(self.env.get_build_dir()) / subdir)
-        self.generated_raw = [rel_path(x, False, True) for x in self.generated_raw]
-        self.includes = list(OrderedSet([rel_path(x, True, False) for x in OrderedSet(self.includes)] + [build_dir_rel]))
-        self.sys_includes = list(OrderedSet([rel_path(x, True, False) for x in OrderedSet(self.sys_includes)]))
-        self.sources = [rel_path(x, False, False) for x in self.sources]
+        self.generated_raw = non_optional(rel_path(x, False, True) for x in self.generated_raw)
+        self.includes = non_optional(itertools.chain((rel_path(x, True, False) for x in OrderedSet(self.includes)), [build_dir_rel]))
+        self.sys_includes = non_optional(rel_path(x, True, False) for x in OrderedSet(self.sys_includes))
+        self.sources = non_optional(rel_path(x, False, False) for x in self.sources)
 
         # Resolve custom targets
         for gen_file in self.generated_raw: