Cleanup: avoid constructing std::string from nullptr

This probably never in practice in these cases.
Constructing a `std::string` from nullptr is invalid.
Starting with C++23, the `nullptr_t` is even explicitly deleted.

source/blender/blenkernel/intern/key.cc

@@ -1958,7 +1958,7 @@ void BKE_keyblock_copy_settings(KeyBlock *kb_dst, const KeyBlock *kb_src)
 std::optional<std::string> BKE_keyblock_curval_rnapath_get(const Key *key, const KeyBlock *kb)
 {
   if (ELEM(nullptr, key, kb)) {
-    return nullptr;
+    return std::nullopt;
   }
   PointerRNA ptr = RNA_pointer_create((ID *)&key->id, &RNA_ShapeKey, (KeyBlock *)kb);
   PropertyRNA *prop = RNA_struct_find_property(&ptr, "value");

source/blender/editors/space_node/node_draw.cc

@@ -231,14 +231,15 @@ static const char *node_socket_get_translation_context(const bNodeSocket &socket
     return nullptr;
   }
 
-  blender::StringRefNull translation_context = socket.runtime->declaration->translation_context;
+  const std::optional<std::string> &translation_context =
+      socket.runtime->declaration->translation_context;
 
   /* Default context. */
-  if (translation_context.is_empty()) {
+  if (!translation_context.has_value()) {
     return nullptr;
   }
 
-  return translation_context.data();
+  return translation_context->c_str();
 }
 
 static void node_socket_add_tooltip_in_node_editor(const bNodeSocket &sock, uiLayout &layout);

source/blender/nodes/NOD_node_declaration.hh

@@ -173,7 +173,7 @@ class SocketDeclaration : public ItemDeclaration {
   std::string short_label;
   std::string identifier;
   std::string description;
-  std::string translation_context;
+  std::optional<std::string> translation_context;
   /** Defined by whether the socket is part of the node's input or
    * output socket declaration list. Included here for convenience. */
   eNodeSocketInOut in_out;
@@ -284,7 +284,8 @@ class BaseSocketDeclarationBuilder {
 
   BaseSocketDeclarationBuilder &description(std::string value = "");
 
-  BaseSocketDeclarationBuilder &translation_context(std::string value = BLT_I18NCONTEXT_DEFAULT);
+  BaseSocketDeclarationBuilder &translation_context(
+      std::optional<std::string> value = std::nullopt);
 
   BaseSocketDeclarationBuilder &no_muted_links(bool value = true);
 
@@ -437,7 +438,7 @@ class PanelDeclaration : public ItemDeclaration {
   int identifier;
   std::string name;
   std::string description;
-  std::string translation_context;
+  std::optional<std::string> translation_context;
   bool default_collapsed = false;
   Vector<ItemDeclaration *> items;
   /** Index in the list of panels on the node. */

source/blender/nodes/intern/node_declaration.cc

@@ -586,7 +586,8 @@ BaseSocketDeclarationBuilder &BaseSocketDeclarationBuilder::description(std::str
   return *this;
 }
 
-BaseSocketDeclarationBuilder &BaseSocketDeclarationBuilder::translation_context(std::string value)
+BaseSocketDeclarationBuilder &BaseSocketDeclarationBuilder::translation_context(
+    std::optional<std::string> value)
 {
   decl_base_->translation_context = std::move(value);
   return *this;