Introduce `mapNullable` into lib and use it in a few places

Also simply some configure flag logic my grep also alerted me too.

lib/trivial.nix

···
   # Flip the order of the arguments of a binary function.
   flip = f: a: b: f b a;
 
+  # Apply function if argument is non-null
+  mapNullable = f: a: if isNull a then a else f a;
+
   # Pull in some builtins not included elsewhere.
   inherit (builtins)
     pathExists readFile isBool isFunction

nixos/modules/services/monitoring/graphite.nix

···
 
 let
   cfg = config.services.graphite;
-  writeTextOrNull = f: t: if t == null then null else pkgs.writeTextDir f t;
+  writeTextOrNull = f: t: mapNullable (pkgs.writeTextDir f) t;
 
   dataDir = cfg.dataDir;
 

nixos/modules/services/monitoring/prometheus/default.nix

···
           };
         });
         default = null;
-        apply = x: if x == null then null else _filter x;
+        apply = x: mapNullable _filter x;
         description = ''
           Optional http login credentials for metrics scraping.
         '';

pkgs/development/libraries/wiredtiger/default.nix

···
 
 with stdenv.lib;
 let
-  mkFlag = trueStr: falseStr: cond: name: val:
-    if cond == null then null else
-      "--${if cond != false then trueStr else falseStr}${name}${if val != null && cond != false then "=${val}" else ""}";
+  mkFlag = trueStr: falseStr: cond: name: val: "--"
+    + (if cond then trueStr else falseStr)
+    + name
+    + optionalString (val != null && cond != false) "=${val}";
   mkEnable = mkFlag "enable-" "disable-";
   mkWith = mkFlag "with-" "without-";
   mkOther = mkFlag "" "" true;

pkgs/os-specific/linux/nvidia-x11/generic.nix

···
         withGtk2 = preferGtk2;
         withGtk3 = !preferGtk2;
       };
-      persistenced = if persistencedSha256 == null then null else callPackage (import ./persistenced.nix self persistencedSha256) { };
+      persistenced = mapNullable (hash: callPackage (import ./persistenced.nix self hash) { }) persistencedSha256;
     };
 
     meta = with stdenv.lib; {

pkgs/servers/shishi/default.nix

···
 }:
 
 let
-  mkFlag = trueStr: falseStr: cond: name: val:
-    if cond == null then null else
-      "--${if cond != false then trueStr else falseStr}${name}${if val != null && cond != false then "=${val}" else ""}";
+  mkFlag = trueStr: falseStr: cond: name: val: "--"
+    + (if cond then trueStr else falseStr)
+    + name
+    + stdenv.lib.optionalString (val != null && cond != false) "=${val}";
   mkEnable = mkFlag "enable-" "disable-";
   mkWith = mkFlag "with-" "without-";
   mkOther = mkFlag "" "" true;

pkgs/tools/filesystems/ceph/generic.nix

···
 with stdenv.lib;
 let
   inherit (python2Packages) python;
-  mkFlag = trueStr: falseStr: cond: name: val:
-    if cond == null then null else
-      "--${if cond != false then trueStr else falseStr}${name}"
-      + "${if val != null && cond != false then "=${val}" else ""}";
-
+  mkFlag = trueStr: falseStr: cond: name: val: "--"
+    + (if cond then trueStr else falseStr)
+    + name
+    + optionalString (val != null && cond != false) "=${val}";
   mkEnable = mkFlag "enable-" "disable-";
   mkWith = mkFlag "with-" "without-";
   mkOther = mkFlag "" "" true;