refactor: Add simple clang-tidy readability checks (#6556)

This change enables the following clang-tidy checks:
-  readability-avoid-nested-conditional-operator,
-  readability-avoid-return-with-void-value,
-  readability-braces-around-statements,
-  readability-const-return-type,
-  readability-container-contains,
-  readability-container-size-empty,
-  readability-else-after-return,
-  readability-make-member-function-const,
-  readability-redundant-casting,
-  readability-redundant-inline-specifier,
-  readability-redundant-member-init,
-  readability-redundant-string-init,
-  readability-reference-to-constructed-temporary,
-  readability-static-definition

src/test/protocol/SecretKey_test.cpp

@@ -112,7 +112,7 @@ public:
 
                 auto sig = signDigest(pk, sk, digest);
 
-                BEAST_EXPECT(sig.size() != 0);
+                BEAST_EXPECT(!sig.empty());
                 BEAST_EXPECT(verifyDigest(pk, digest, sig, true));
 
                 // Wrong digest:
@@ -150,7 +150,7 @@ public:
 
                 auto sig = sign(pk, sk, makeSlice(data));
 
-                BEAST_EXPECT(sig.size() != 0);
+                BEAST_EXPECT(!sig.empty());
                 BEAST_EXPECT(verify(pk, makeSlice(data), sig));
 
                 // Construct wrong data: