[core] Let StringRef carry a null pointer at zero length

The generated api messages start their encode only string fields that way. starts_with,
compare and str() no longer hand a null pointer to memcmp or the std::string range
constructor when there is nothing to compare or copy, the class comment states the
contract, and gtest cases pin every member on a null, empty view.

esphome/core/string_ref.h

@@ -22,6 +22,10 @@ namespace esphome {
  * pointer.  When it is default constructed, it has empty string.  You can freely copy or move around this struct, but
  * never free its pointer.  str() function can be used to export the content as std::string. StringRef is adopted from
  * <https://github.com/nghttp2/nghttp2/blob/29cbf8b83ff78faf405d1086b16adc09a8772eca/src/template.h#L376>
+ *
+ * A StringRef may carry a null pointer while its length is zero (the generated api messages start their encode only
+ * string fields that way). Every member treats that as the empty string; only c_str() hands the null pointer on, so
+ * callers that print or copy through c_str() must check empty() first.
  */
 class StringRef {
  public:
@@ -78,7 +82,7 @@ class StringRef {
 
   /// True if the view begins with the given prefix (std::string::starts_with-like)
   bool starts_with(const StringRef &prefix) const {
-    return len_ >= prefix.len_ && std::memcmp(base_, prefix.base_, prefix.len_) == 0;
+    return len_ >= prefix.len_ && (prefix.len_ == 0 || std::memcmp(base_, prefix.base_, prefix.len_) == 0);
   }
   bool starts_with(const char *prefix) const { return this->starts_with(StringRef(prefix)); }
   bool starts_with(const std::string &prefix) const { return this->starts_with(StringRef(prefix)); }
@@ -92,14 +96,15 @@ class StringRef {
     return actual;
   }
 
-  std::string str() const { return std::string(base_, len_); }
+  std::string str() const { return len_ == 0 ? std::string() : std::string(base_, len_); }
   const uint8_t *byte() const { return reinterpret_cast<const uint8_t *>(base_); }
 
   operator std::string() const { return str(); }
 
   /// Compare (compatible with std::string::compare)
   int compare(const StringRef &other) const {
-    int result = std::memcmp(base_, other.base_, std::min(len_, other.len_));
+    size_type common = std::min(len_, other.len_);
+    int result = common == 0 ? 0 : std::memcmp(base_, other.base_, common);
     if (result != 0)
       return result;
     if (len_ < other.len_)

tests/components/core/test_string_ref.cpp

@@ -59,4 +59,39 @@ TEST(StringRefStartsWith, RefOverloadComparesOnlyTheViewedLength) {
   EXPECT_TRUE(ref.starts_with(prefix));
 }
 
+// The generated api messages start their encode only string fields as a null pointer with zero
+// length; every member must treat that exactly like the default constructed empty string.
+TEST(StringRefNullEmpty, BehavesAsEmptyString) {
+  const StringRef null_empty{nullptr, 0};
+  const StringRef empty;
+  EXPECT_TRUE(null_empty.empty());
+  EXPECT_EQ(null_empty.size(), 0u);
+  EXPECT_EQ(null_empty.c_str(), nullptr);
+  EXPECT_TRUE(null_empty == empty);
+  EXPECT_TRUE(null_empty == "");
+  EXPECT_TRUE(null_empty == std::string());
+  EXPECT_EQ(null_empty.compare(empty), 0);
+  EXPECT_EQ(null_empty.compare(""), 0);
+  EXPECT_LT(null_empty.compare("a"), 0);
+  EXPECT_TRUE(null_empty.starts_with(""));
+  EXPECT_FALSE(null_empty.starts_with("a"));
+  EXPECT_EQ(null_empty.str(), std::string());
+  EXPECT_EQ(null_empty.substr(0), std::string());
+  EXPECT_EQ(null_empty.find('a'), std::string::npos);
+  EXPECT_EQ(null_empty.find("a"), std::string::npos);
+  char buf[4] = "xyz";
+  EXPECT_EQ(null_empty.copy(buf, sizeof(buf)), 0u);
+  EXPECT_EQ(null_empty.begin(), null_empty.end());
+}
+
+TEST(StringRefNullEmpty, ComparesAgainstText) {
+  const StringRef null_empty{nullptr, 0};
+  const StringRef text("abc", 3);
+  EXPECT_FALSE(null_empty == text);
+  EXPECT_FALSE(text == null_empty);
+  EXPECT_LT(null_empty.compare(text), 0);
+  EXPECT_GT(text.compare(null_empty), 0);
+  EXPECT_TRUE(text.starts_with(null_empty));
+}
+
 }  // namespace esphome::core::testing