[RGen] Update string strategy to fail on empty selectors. (#23646)

Do not allow selectors to be null or empty.

Author: mandel-macaque

src/rgen/Microsoft.Macios.Bindings.Analyzer/Validators/ExportMethodAttributeValidator.cs

@@ -231,7 +231,7 @@ internal static bool FlagsAreValid (ExportMethod exportData, RootContext context
 	/// <returns><c>true</c> if the data is valid; otherwise, <c>false</c>.</returns>
 	internal static bool SelectorIsNotNull (string? selector, RootContext context, out ImmutableArray<Diagnostic> diagnostics,
 		Location? location = null)
-		=> StringStrategies.IsNotNull (
+		=> StringStrategies.IsNotNullOrEmpty (
 			selector: selector,
 			descriptor: RBI0022, // A export property must have a selector defined
 			diagnostics: out diagnostics,

src/rgen/Microsoft.Macios.Bindings.Analyzer/Validators/ExportPropertyAttributeValidator.cs

@@ -25,7 +25,7 @@ class ExportPropertyAttributeValidator : Validator<ExportData<Property>> {
 	/// <returns><c>true</c> if the data is valid; otherwise, <c>false</c>.</returns>
 	internal static bool SelectorIsNotNull (string? selector, RootContext context, out ImmutableArray<Diagnostic> diagnostics,
 		Location? location = null)
-		=> StringStrategies.IsNotNull (
+		=> StringStrategies.IsNotNullOrEmpty (
 			selector: selector,
 			descriptor: RBI0018, // A export property must have a selector defined
 			diagnostics: out diagnostics,

src/rgen/Microsoft.Macios.Bindings.Analyzer/Validators/FieldValidator.cs

@@ -25,7 +25,7 @@ class FieldValidator : Validator<Property> {
 	/// <returns><c>true</c> if the selector is not null or empty; otherwise, <c>false</c>.</returns>
 	internal static bool SelectorIsNotNull (string? selector, RootContext context, out ImmutableArray<Diagnostic> diagnostics,
 		Location? location = null)
-		=> StringStrategies.IsNotNull (
+		=> StringStrategies.IsNotNullOrEmpty (
 			selector: selector,
 			descriptor: RBI0018, // A export property must have a selector defined
 			diagnostics: out diagnostics,

src/rgen/Microsoft.Macios.Bindings.Analyzer/Validators/StringStrategies.cs

@@ -17,11 +17,11 @@ public static class StringStrategies {
 	/// <param name="diagnostics">When this method returns, contains an array of diagnostics if the data is invalid; otherwise, an empty array.</param>
 	/// <param name="location">The code location to be used for the diagnostics.</param>
 	/// <returns><c>true</c> if the data is valid; otherwise, <c>false</c>.</returns>
-	internal static bool IsNotNull (string? selector, DiagnosticDescriptor descriptor, out ImmutableArray<Diagnostic> diagnostics,
+	internal static bool IsNotNullOrEmpty (string? selector, DiagnosticDescriptor descriptor, out ImmutableArray<Diagnostic> diagnostics,
 		Location? location = null)
 	{
 		diagnostics = ImmutableArray<Diagnostic>.Empty;
-		if (selector is not null)
+		if (!string.IsNullOrEmpty (selector))
 			return true;
 		diagnostics = [
 			Diagnostic.Create (

tests/rgen/Microsoft.Macios.Bindings.Analyzer.Tests/Validators/FieldValidatorTests.cs

@@ -154,7 +154,7 @@ public void ValidateValidFieldPropertyTests ()
 
 	[Theory]
 	[InlineData ("validSelector", true, 0)]
-	[InlineData ("", true, 0)]
+	[InlineData ("", false, 1)]
 	[InlineData (null, false, 1)]
 	[InlineData ("another_valid_selector", true, 0)]
 	[InlineData ("valid123", true, 0)]