clang-tidy: remove pointless string init

Found with readability-redundant-string-init

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

samples/Jzon.cpp

@@ -1011,10 +1011,10 @@ namespace Jzon
 	{
 		std::stack<std::pair<std::string, Node*> > nodeStack;
 
-		std::string name = "";
+        std::string name;
 
-		Token token;
-		while (!tokens.empty())
+        Token token;
+        while (!tokens.empty())
 		{
 			token = tokens.front();
 			tokens.pop();

src/exiv2.cpp

@@ -1518,7 +1518,7 @@ namespace {
 
     std::string parseEscapes(const std::string& input)
     {
-        std::string result = "";
+        std::string result;
         for (unsigned int i = 0; i < input.length(); ++i) {
             char ch = input[i];
             if (ch != '\\') {
@@ -1571,7 +1571,7 @@ namespace {
                         break;
                     }
 
-                    std::string ucs2toUtf8 = "";
+                    std::string ucs2toUtf8;
                     ucs2toUtf8.push_back((char) ((acc & 0xff00) >> 8));
                     ucs2toUtf8.push_back((char) (acc & 0x00ff));
 

src/image.cpp

@@ -376,7 +376,7 @@ namespace Exiv2 {
                     throw Error(kerInvalidTypeValue);
                 }
 
-                std::string sp  = "" ; // output spacer
+                std::string sp;  // output spacer
 
                 //prepare to print the value
                 uint32_t kount  = isPrintXMP(tag,option) ? count // haul in all the data

src/olympusmn_int.cpp

@@ -1481,7 +1481,7 @@ namespace Exiv2 {
 
             // If value 2 is present, it is used instead of value 1.
             if (value.count() > 1) {
-                std::string p = "";    // Used to enable ',' separation
+                std::string p;  // Used to enable ',' separation
 
                 v = (uint16_t)value.toLong(1);
                 for (int i = 0; focusModes1[i].val != 0; i++) {

src/pngimage.cpp

@@ -271,7 +271,7 @@ namespace Exiv2 {
                 // format output
                 const int    iMax = 30 ;
                 const uint32_t blen = dataOffset > iMax ? iMax : dataOffset ;
-                std::string dataString = "";
+                std::string dataString;
                 // if blen == 0 => slice construction fails
                 if (blen > 0) {
                     std::stringstream ss;

src/xmp.cpp

@@ -502,7 +502,7 @@ namespace Exiv2 {
             std::map<std::string,std::string>* p = (std::map<std::string,std::string>*) refCon;
             std::map<std::string,std::string>& m = *p;
 
-            std::string b("");
+            std::string b;
             if ( bNS ) {  // store the NS in dict[""]
                 m[b]=out;
             } else if ( m.find(b) != m.end() ) {  // store dict[uri] = dict[""]