Don't process language if block language not set

Adding this guard to the function prevents an error if the language is undefined. `env.language.substring(0, 1).toUpperCase()` can fail with `Uncaught TypeError: Cannot read property 'substring' of undefined` when `env.language` is undefined.

plugins/show-language/prism-show-language.js

@@ -14,7 +14,7 @@ if (!Prism.plugins.toolbar) {
 var Languages = /*languages_placeholder[*/{"html":"HTML","xml":"XML","svg":"SVG","mathml":"MathML","css":"CSS","clike":"C-like","javascript":"JavaScript","abap":"ABAP","actionscript":"ActionScript","apacheconf":"Apache Configuration","apl":"APL","applescript":"AppleScript","arff":"ARFF","asciidoc":"AsciiDoc","asm6502":"6502 Assembly","aspnet":"ASP.NET (C#)","autohotkey":"AutoHotkey","autoit":"AutoIt","basic":"BASIC","csharp":"C#","cpp":"C++","coffeescript":"CoffeeScript","csp":"Content-Security-Policy","css-extras":"CSS Extras","django":"Django/Jinja2","erb":"ERB","fsharp":"F#","gedcom":"GEDCOM","glsl":"GLSL","graphql":"GraphQL","http":"HTTP","hpkp":"HTTP Public-Key-Pins","hsts":"HTTP Strict-Transport-Security","ichigojam":"IchigoJam","inform7":"Inform 7","json":"JSON","latex":"LaTeX","livescript":"LiveScript","lolcode":"LOLCODE","markup-templating":"Markup templating","matlab":"MATLAB","mel":"MEL","n4js":"N4JS","nasm":"NASM","nginx":"nginx","nsis":"NSIS","objectivec":"Objective-C","ocaml":"OCaml","opencl":"OpenCL","parigp":"PARI/GP","php":"PHP","php-extras":"PHP Extras","plsql":"PL/SQL","powershell":"PowerShell","properties":".properties","protobuf":"Protocol Buffers","q":"Q (kdb+ database)","jsx":"React JSX","tsx":"React TSX","renpy":"Ren'py","rest":"reST (reStructuredText)","sas":"SAS","sass":"Sass (Sass)","scss":"Sass (Scss)","sql":"SQL","soy":"Soy (Closure Template)","typescript":"TypeScript","vbnet":"VB.Net","vhdl":"VHDL","vim":"vim","visual-basic":"Visual Basic","wasm":"WebAssembly","wiki":"Wiki markup","xojo":"Xojo (REALbasic)","yaml":"YAML"}/*]*/;
 Prism.plugins.toolbar.registerButton('show-language', function(env) {
 	var pre = env.element.parentNode;
-	if (!pre || !/pre/i.test(pre.nodeName)) {
+	if (!pre || !/pre/i.test(pre.nodeName) || !env.language) {
 		return;
 	}
 	var language = pre.getAttribute('data-language') || Languages[env.language] || (env.language.substring(0, 1).toUpperCase() + env.language.substring(1));