Handle non-existing files in find_block_in_file (#1002)

Currently find_block_in_file will throw an error if the file does not
exist, which can happen if a Base docstring is being included.

CHANGELOG.md

@@ -1,5 +1,9 @@
 # Documenter.jl changelog
 
+## Version `v0.22.4`
+
+* ![Bugfix][badge-bugfix] Documenter no longer crashes if the build includes doctests from docstrings that are defined in files that do not exist on the file system (e.g. if a Julia Base docstring is included when running a non-source Julia build). ([#1002][github-1002])
+
 ## Version `v0.22.3`
 
 * ![Bugfix][badge-bugfix] Fixed filepaths for images included in the .tex file for PDF output on Windows. ([#999][github-999])
@@ -299,6 +303,7 @@
 [github-995]: https://github.com/JuliaDocs/Documenter.jl/pull/995
 [github-996]: https://github.com/JuliaDocs/Documenter.jl/pull/996
 [github-999]: https://github.com/JuliaDocs/Documenter.jl/pull/999
+[github-1002]: https://github.com/JuliaDocs/Documenter.jl/pull/1002
 
 [documenterlatex]: https://github.com/JuliaDocs/DocumenterLaTeX.jl
 [documentermarkdown]: https://github.com/JuliaDocs/DocumenterMarkdown.jl

src/Utilities/Utilities.jl

@@ -16,6 +16,7 @@ regex_escape(str) = sprint(escape_string, str, "\\^\$.|?*+()[{")
 function find_block_in_file(code, file)
     source_file = Base.find_source_file(file)
     source_file === nothing && return nothing
+    isfile(source_file) || return nothing
     content = read(source_file, String)
     content = replace(content, "\r\n" => "\n")
     # make a regex of the code that matches leading whitespace