[layout] Mark language as nullable

GI needs it apparently.

src/hb-ot-layout.cc

@@ -2100,7 +2100,7 @@ hb_ot_layout_get_font_extents (hb_font_t         *font,
  * @font: a font
  * @direction: text direction.
  * @script:  script.
- * @language: language.
+ * @language: (nullable): language.
  * @extents: (out) (nullable): font extents if found.
  *
  * Fetches script/language-specific font extents.  These values are
@@ -2229,7 +2229,7 @@ hb_ot_layout_get_baseline (hb_font_t                   *font,
  * @baseline_tag: a baseline tag
  * @direction: text direction.
  * @script:  script.
- * @language: language, currently unused.
+ * @language: (nullable): language, currently unused.
  * @coord: (out) (nullable): baseline value if found.
  *
  * Fetches a baseline value from the face.
@@ -2484,7 +2484,7 @@ hb_ot_layout_get_baseline_with_fallback (hb_font_t                   *font,
  * @baseline_tag: a baseline tag
  * @direction: text direction.
  * @script:  script.
- * @language: language, currently unused.
+ * @language: (nullable): language, currently unused.
  * @coord: (out): baseline value if found.
  *
  * Fetches a baseline value from the face, and synthesizes

src/hb-ot-tag.cc

@@ -412,7 +412,7 @@ parse_private_use_subtag (const char     *private_use_subtag,
 /**
  * hb_ot_tags_from_script_and_language:
  * @script: an #hb_script_t to convert.
- * @language: an #hb_language_t to convert.
+ * @language: (nullable): an #hb_language_t to convert.
  * @script_count: (inout) (optional): maximum number of script tags to retrieve (IN)
  * and actual number of script tags retrieved (OUT)
  * @script_tags: (out) (optional): array of size at least @script_count to store the