Replace 0 and NULL with nullptr where apropriate.

- fixed using clang-tidy's modernize-use-nullptr check.

glabels/Merge/Factory.cpp

@@ -102,7 +102,7 @@ namespace glabels
 		///
 		void Factory::init()
 		{
-			static Factory* singletonInstance = 0;
+			static Factory* singletonInstance = nullptr;
 			if ( !singletonInstance )
 			{
 				singletonInstance = new Factory();

glabels/Merge/Merge.cpp

@@ -95,7 +95,7 @@ namespace glabels
 			mRecordList.clear();
 
 			open();
-			for ( Record* record = readNextRecord(); record != 0; record = readNextRecord() )
+			for ( Record* record = readNextRecord(); record != nullptr; record = readNextRecord() )
 			{
 				mRecordList.append( record );
 			}

glabels/Merge/None.cpp

@@ -119,7 +119,7 @@ namespace glabels
 		///
 		Record* None::readNextRecord()
 		{
-			return 0;
+			return nullptr;
 		}
 
 	}

glabels/Merge/Text.cpp

@@ -139,7 +139,7 @@ namespace glabels
 
 				return record;
 			}
-			return 0;
+			return nullptr;
 		}