[Version parsing] Fix precondition failure with no characters in between .'s

Uncovered during compiler testing.

Author: DougGregor

Sources/SwiftIfConfig/VersionTuple+Parsing.swift

@@ -27,7 +27,7 @@ extension VersionTuple {
     components = []
 
     // Version value are separated by periods.
-    let componentStrings = versionString.split(separator: ".")
+    let componentStrings = versionString.split(separator: ".", omittingEmptySubsequences: false)
 
     /// Record a component after checking its value.
     func recordComponent(_ value: Int) throws {

Sources/SwiftIfConfig/VersionTuple.swift

@@ -33,7 +33,7 @@ public struct VersionTuple: Sendable {
   public init?(parsing string: String) {
     self.components = []
 
-    for componentText in string.split(separator: ".") {
+    for componentText in string.split(separator: ".", omittingEmptySubsequences: false) {
       guard let component = Int(componentText) else {
         return nil
       }

Tests/SwiftIfConfigTest/EvaluateTests.swift

@@ -176,6 +176,17 @@ public class EvaluateTests: XCTestCase {
     assertIfConfig("swift(>=5.5)", .active)
     assertIfConfig("swift(<6)", .active)
     assertIfConfig("swift(>=6)", .unparsed)
+    assertIfConfig(
+      "swift(>=...)",
+      .unparsed,
+      diagnostics: [
+        DiagnosticSpec(
+          message: "'swift' version check has invalid version ''",
+          line: 1,
+          column: 9
+        )
+      ]
+    )
     assertIfConfig("compiler(>=5.8)", .active)
     assertIfConfig("compiler(>=5.9)", .active)
     assertIfConfig("compiler(>=5.10)", .unparsed)
@@ -206,6 +217,17 @@ public class EvaluateTests: XCTestCase {
         )
       ]
     )
+    assertIfConfig(
+      #"_compiler_version("...")"#,
+      .unparsed,
+      diagnostics: [
+        DiagnosticSpec(
+          message: "found empty version component",
+          line: 1,
+          column: 20
+        )
+      ]
+    )
   }
 
   func testCanImport() throws {