Remove extraneous newline for empty abstract (#161)

In the event a configuration has an empty abstract and non-empty discussion,
an extra line was generated. This patch removes that line.

ISSUE: SAP-156 #156

Author: erica

Sources/ArgumentParser/Usage/HelpGenerator.swift

@@ -117,7 +117,10 @@ internal struct HelpGenerator {
 
     self.abstract = currentCommand.configuration.abstract
     if !currentCommand.configuration.discussion.isEmpty {
-      self.abstract += "\n\n\(currentCommand.configuration.discussion)"
+      if !self.abstract.isEmpty {
+        self.abstract += "\n"
+      }
+      self.abstract += "\n\(currentCommand.configuration.discussion)"
     }
 
     self.usage = Usage(components: [usageString])

Tests/ArgumentParserUnitTests/HelpGenerationTests.swift

@@ -298,4 +298,23 @@ extension HelpGenerationTests {
     """)
 
   }
+  
+  struct J: ParsableCommand {
+    static var configuration = CommandConfiguration(discussion: "test")
+  }
+  
+  func testOverviewButNoAbstractSpacing() {
+    let renderedHelp = HelpGenerator(J.self).rendered()
+    AssertEqualStringsIgnoringTrailingWhitespace(renderedHelp, """
+    OVERVIEW:
+    test
+
+    USAGE: j
+    
+    OPTIONS:
+      -h, --help              Show help information.
+    
+    """)
+  }
+
 }