[log_formats] ignore empty lines in non-multiline formats

src/log_format.cc

@@ -904,6 +904,10 @@ void external_log_format::annotate(uint64_t line_number, shared_buffer_ref &line
         return;
     }
 
+    if (line.empty()) {
+        return;
+    }
+
     int pat_index = this->pattern_index_for_line(line_number);
     pattern &pat = *this->elf_pattern_order[pat_index];
 

src/shared_buffer.hh

@@ -73,7 +73,9 @@ public:
         return *this;
     };
 
-    bool empty() const { return this->sb_data == nullptr; };
+    bool empty() const {
+        return this->sb_data == nullptr || this->sb_length == 0;
+    };
 
     const char *get_data() const { return this->sb_data; };