Fix error loading "text" syntax for hover (#4574)

rust-analyzer sometimes returns a hover result with language being
"text", but there's no syntax/text.vim, so this would fail with:

    Error detected while processing function <SNR>150_VimOutputCallback[6]..<lambda>8[1]..ale#lsp#HandleMessage[30]..ale#hover#HandleLSPResponse[42]..ale#floating_preview#Show[13]..<SNR>161_VimShow:
    line   13:
    E484: Cannot open file syntax/text.vim

Only including the file when it actually exists fixes this.

autoload/ale/hover.vim

@@ -70,6 +70,18 @@ function! s:ConvertLanguageName(language) abort
     return a:language
 endfunction
 
+" Cache syntax file (non-)existence to avoid calling globpath repeatedly.
+let s:syntax_file_exists_cache = {}
+
+function! s:SyntaxFileExists(syntax_file) abort
+    if !has_key(s:syntax_file_exists_cache, a:syntax_file)
+        let s:syntax_file_exists_cache[a:syntax_file] =
+        \   !empty(globpath(&runtimepath, a:syntax_file))
+    endif
+
+    return s:syntax_file_exists_cache[a:syntax_file]
+endfunction
+
 function! ale#hover#ParseLSPResult(contents) abort
     let l:includes = {}
     let l:highlights = []
@@ -160,10 +172,11 @@ function! ale#hover#ParseLSPResult(contents) abort
                 let l:language = s:ConvertLanguageName(l:language)
 
                 if !empty(l:language)
-                    let l:includes[l:language] = printf(
-                    \   'syntax/%s.vim',
-                    \   l:language,
-                    \)
+                    let l:syntax_file = printf('syntax/%s.vim', l:language)
+
+                    if s:SyntaxFileExists(l:syntax_file)
+                        let l:includes[l:language] = l:syntax_file
+                    endif
 
                     let l:start = len(l:lines) + 1
                     let l:end = l:start + len(l:marked_lines)