Null check annotations for Semantic Tokens API

[BoykoAlex]

The downstream plugins using the new ISemanticTokensProvider API and having JDT null analysis turned on having issues compiling classes deriving from the ISemanticTokensProvider. Whether I add @Nullable or @NonNull the class still fails to compile with

Error: 2,352 [ERROR] Failed to execute goal org.eclipse.tycho:tycho-compiler-plugin:4.0.10:compile (default-compile) on project org.eclipse.lsp4e.jdt: Compilation failure: Compilation failure: 
Error: 2,352 [ERROR] /Users/runner/work/lsp4e/lsp4e/org.eclipse.lsp4e.jdt/src/org/eclipse/lsp4e/jdt/LSJavaSemanticTokensProvider.java:[42] 
Error: 2,352 [ERROR] 	public Collection<ISemanticTokensProvider.SemanticToken> computeSemanticTokens(CompilationUnit ast) {
Error: 2,352 [ERROR] 	       ^^^^^^^^^^
Error: 2,352 [ERROR] The return type is incompatible with 'Collection<ISemanticTokensProvider.SemanticToken>' returned from ISemanticTokensProvider.computeSemanticTokens(CompilationUnit) (mismatching null constraints)
Error: 2,352 [ERROR] 1 problem (1 error)

Adding the @NonNull for the return type and the parameter sets the proper API expectations as well as hopefully would help sorting the downstream compiler errors out.