fix(telemetry): Do not build a string_view from a null attribute pointer

The C-string overload of setAttribute handed its pointer straight to
std::string_view, whose pointer constructor reads the pointer to find the
length. A null one is undefined behaviour.

A null pointer carries no text, and an empty value already means something on
this class, so nothing is recorded rather than an empty string.

The overload itself has to stay. Without it a string literal would bind to the
bool overload, because pointer-to-bool is a standard conversion and beats the
string_view one, so every string attribute would silently record true. The
header now records that.

include/xrpl/telemetry/SpanGuard.h

@@ -447,7 +447,14 @@ public:
     setAttribute(std::string_view key, std::string_view value) noexcept;
 
     /**
-     * Set a string attribute (C-string overload). No-op on a null guard.
+     * Set a string attribute from a C string. No-op on a null guard.
+     *
+     * @param key   Attribute key.
+     * @param value Null-terminated text. A null pointer records nothing, since
+     * an empty value is already a meaningful value here.
+     * @note This overload is required, not a convenience. Without it a string
+     * literal binds to the bool overload, because pointer-to-bool is a standard
+     * conversion and beats the std::string_view one.
      */
     void
     setAttribute(std::string_view key, char const* value) noexcept;
@@ -739,7 +746,14 @@ public:
     setAttribute(std::string_view key, std::string_view value) noexcept;
 
     /**
-     * Set a string attribute (C-string overload). No-op on a null guard.
+     * Set a string attribute from a C string. No-op on a null guard.
+     *
+     * @param key   Attribute key.
+     * @param value Null-terminated text. A null pointer records nothing, since
+     * an empty value is already a meaningful value here.
+     * @note This overload is required, not a convenience. Without it a string
+     * literal binds to the bool overload, because pointer-to-bool is a standard
+     * conversion and beats the std::string_view one.
      */
     void
     setAttribute(std::string_view key, char const* value) noexcept;

src/libxrpl/telemetry/SpanGuard.cpp

@@ -408,7 +408,11 @@ SpanGuard::setAttribute(std::string_view key, std::string_view value) noexcept
 void
 SpanGuard::setAttribute(std::string_view key, char const* value) noexcept
 {
-    setAttribute(key, std::string_view(value));
+    // A std::string_view built from a pointer reads that pointer to find its
+    // length, so a null one is undefined behaviour. A null pointer carries no
+    // text, and an empty value already means something here, so record nothing.
+    if (value != nullptr)
+        setAttribute(key, std::string_view(value));
 }
 
 void