Make Shader::addSource() no-op for empty string.

src/Shader.cpp

@@ -80,6 +80,8 @@ Shader& Shader::operator=(Shader&& other) {
 }
 
 Shader& Shader::addSource(const std::string& source) {
+    if(source.empty()) return *this;
+
     if(_state == State::Initialized)
         /* Fix line numbers, so line 41 of third added file is marked as 3(41).
            Source 0 is the #version string added in constructor. */

src/Shader.h

@@ -139,7 +139,7 @@ class MAGNUM_EXPORT Shader {
          *
          * If the shader is not compiled already, adds given source to source
          * list. Note that it is possible to compile shader from more than
-         * one source.
+         * one source. If passed string is empty, the function does nothing.
          * @see addFile()
          */
         Shader& addSource(const std::string& source);