Address readability-redundant-string-init warnings.

The warnings were:

```
/w/dolphin-memory-engine/Source/GUI/MemCopy/DlgCopy.cpp:230:15: warning: redundant string initialization [readability-redundant-string-init]
  230 |   std::string beforeAll = "";
      |               ^~~~~~~~~~~~~~
      |               beforeAll
/w/dolphin-memory-engine/Source/GUI/MemCopy/DlgCopy.cpp:231:15: warning: redundant string initialization [readability-redundant-string-init]
  231 |   std::string beforeByte = "";
      |               ^~~~~~~~~~~~~~~
      |               beforeByte
/w/dolphin-memory-engine/Source/GUI/MemCopy/DlgCopy.cpp:232:15: warning: redundant string initialization [readability-redundant-string-init]
  232 |   std::string betweenBytes = "";
      |               ^~~~~~~~~~~~~~~~~
      |               betweenBytes
/w/dolphin-memory-engine/Source/GUI/MemCopy/DlgCopy.cpp:233:15: warning: redundant string initialization [readability-redundant-string-init]
  233 |   std::string afterAll = "";
      |               ^~~~~~~~~~~~~
      |               afterAll
```

Source/GUI/MemCopy/DlgCopy.cpp

@@ -228,10 +228,10 @@ std::string DlgCopy::charToHexString(const char* const input, const size_t count
   std::stringstream ss;
   const char convert[16] = {'0', '1', '2', '3', '4', '5', '6', '7',
                             '8', '9', 'a', 'b', 'c', 'd', 'e', 'f'};
-  std::string beforeAll = "";
-  std::string beforeByte = "";
-  std::string betweenBytes = "";
-  std::string afterAll = "";
+  std::string beforeAll;
+  std::string beforeByte;
+  std::string betweenBytes;
+  std::string afterAll;
 
   switch (format)
   {