try to fix some fanalyzer warnings

Signed-off-by: Rosen Penev <rosenp@gmail.com>

src/convert.cpp

@@ -1284,7 +1284,7 @@ std::string Converter::computeExifDigest(bool tiff) {
 }
 #else
 std::string Converter::computeExifDigest(bool) {
-  return std::string("");
+  return {};
 }
 #endif
 

src/pngchunk_int.cpp

@@ -318,31 +318,25 @@ void PngChunk::parseChunkContent(Image* pImage, const byte* key, size_t keySize,
 }  // PngChunk::parseChunkContent
 
 std::string PngChunk::makeMetadataChunk(const std::string& metadata, MetadataId type) {
-  std::string chunk;
   std::string rawProfile;
 
   switch (type) {
     case mdComment:
-      chunk = makeUtf8TxtChunk("Description", metadata, true);
-      break;
+      return makeUtf8TxtChunk("Description", metadata, true);
     case mdExif:
       rawProfile = writeRawProfile(metadata, "exif");
-      chunk = makeAsciiTxtChunk("Raw profile type exif", rawProfile, true);
-      break;
+      return makeAsciiTxtChunk("Raw profile type exif", rawProfile, true);
     case mdIptc:
       rawProfile = writeRawProfile(metadata, "iptc");
-      chunk = makeAsciiTxtChunk("Raw profile type iptc", rawProfile, true);
-      break;
+      return makeAsciiTxtChunk("Raw profile type iptc", rawProfile, true);
     case mdXmp:
-      chunk = makeUtf8TxtChunk("XML:com.adobe.xmp", metadata, false);
-      break;
+      return makeUtf8TxtChunk("XML:com.adobe.xmp", metadata, false);
     case mdIccProfile:
-      break;
     case mdNone:
-      break;
+      return {};
   }
 
-  return chunk;
+  return {};
 
 }  // PngChunk::makeMetadataChunk
 

src/tags.cpp

@@ -302,7 +302,7 @@ std::string ExifKey::tagLabel() const {
 
 std::string ExifKey::tagDesc() const {
   if (!p_->tagInfo_ || p_->tagInfo_->tag_ == 0xffff)
-    return "";
+    return {};
   return _(p_->tagInfo_->desc_);
 }