Fix nimble run with only one namedBin

Author: iacore

readme.markdown

@@ -871,9 +871,11 @@ file is created instead.
 The binary can be named differently than the source file with the ``namedBin``
 table:
 
+The keys in ``namedBin`` are Nim source filenames (without extension), and the values are executable names.
+
 ```nim
-namedBin["main"] = "mymain"
-namedBin = {"main": "mymain", "main2": "other-main"}.toTable()
+namedBin["main"] = "package-name"
+namedBin = {"main": "package-name", "sourcename2": "other-main"}.toTable()
 ```
 
 Note that `namedBin` entries override duplicates in `bin`.

src/nimblepkg/options.nim

@@ -740,9 +740,9 @@ proc getCompilationBinary*(options: Options, pkgInfo: PackageInfo): Option[strin
       if optRunFile.get("").len > 0:
         optRunFile.get()
       elif pkgInfo.bin.len == 1:
-        toSeq(pkgInfo.bin.values)[0]
+        toSeq(pkgInfo.bin.keys)[0]
       else:
-        ""
+        return none(string)
 
     if runFile.len > 0:
       return some(runFile.changeFileExt(ExeExt))