ValidPathInfo JSON format should use null not omit field

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>
Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

doc/manual/rl-next/store-object-info.md

@@ -0,0 +1,11 @@
+---
+synopsis: Store object info JSON format now uses `null` rather than omitting fields.
+prs: 9995
+---
+
+The [store object info JSON format](@docroot@/protocols/json/store-object-info.md), used for e.g. `nix path-info`, no longer omits fields to indicate absent information, but instead includes the fields with a `null` value.
+For example, `"ca": null` is used to to indicate a store object that isn't content-addressed rather than omitting the `ca` field entirely.
+This makes records of this sort more self-describing, and easier to consume programmatically.
+
+We will follow this design principle going forward;
+the [JSON guidelines](@docroot@/contributing/json-guideline.md) in the contributing section have been updated accordingly.

doc/manual/src/glossary.md

@@ -12,7 +12,7 @@
   For how Nix uses content addresses, see:
 
     - [Content-Addressing File System Objects](@docroot@/store/file-system-object/content-address.md)
-    - [content-addressed store object](#gloss-content-addressed-store-object)
+    - [Content-Addressing Store Objects](@docroot@/store/store-object/content-address.md)
     - [content-addressed derivation](#gloss-content-addressed-derivation)
 
   Software Heritage's writing on [*Intrinsic and Extrinsic identifiers*](https://www.softwareheritage.org/2020/07/09/intrinsic-vs-extrinsic-identifiers) is also a good introduction to the value of content-addressing over other referencing schemes.
@@ -137,9 +137,12 @@
 
 - [content-addressed store object]{#gloss-content-addressed-store-object}
 
-  A [store object] whose [store path] is determined by its contents.
+  A [store object] which is [content-addressed](#gloss-content-address),
+  i.e. whose [store path] is determined by its contents.
   This includes derivations, the outputs of [content-addressed derivations](#gloss-content-addressed-derivation), and the outputs of [fixed-output derivations](#gloss-fixed-output-derivation).
 
+  See [Content-Addressing Store Objects](@docroot@/store/store-object/content-address.md) for details.
+
 - [substitute]{#gloss-substitute}
 
   A substitute is a command invocation stored in the [Nix database] that

doc/manual/src/protocols/json/store-object-info.md

@@ -24,9 +24,11 @@ Info about a [store object].
 
   An array of [store paths][store path], possibly including this one.
 
-* `ca` (optional):
+* `ca`:
 
-  Content address of this store object's file system object, used to compute its store path.
+  If the store object is [content-addressed],
+  this is the content address of this store object's file system object, used to compute its store path.
+  Otherwise (i.e. if it is [input-addressed]), this is `null`.
 
 [store path]: @docroot@/store/store-path.md
 [file system object]: @docroot@/store/file-system-object.md
@@ -37,28 +39,30 @@ Info about a [store object].
 These are not intrinsic properties of the store object.
 In other words, the same store object residing in different store could have different values for these properties.
 
-* `deriver` (optional):
+* `deriver`:
 
-  The path to the [derivation] from which this store object is produced.
+  If known, the path to the [derivation] from which this store object was produced.
+  Otherwise `null`.
 
   [derivation]: @docroot@/glossary.md#gloss-store-derivation
 
 * `registrationTime` (optional):
 
-  When this derivation was added to the store.
+  If known, when this derivation was added to the store.
+  Otherwise `null`.
 
-* `ultimate` (optional):
+* `ultimate`:
 
   Whether this store object is trusted because we built it ourselves, rather than substituted a build product from elsewhere.
 
-* `signatures` (optional):
+* `signatures`:
 
   Signatures claiming that this store object is what it claims to be.
   Not relevant for [content-addressed] store objects,
   but useful for [input-addressed] store objects.
 
-  [content-addressed]: @docroot@/glossary.md#gloss-content-addressed-store-object
-  [input-addressed]: @docroot@/glossary.md#gloss-input-addressed-store-object
+[content-addressed]: @docroot@/store/store-object/content-address.md
+[input-addressed]: @docroot@/glossary.md#gloss-input-addressed-store-object
 
 ### `.narinfo` extra fields