CredsStore validation for RegistryAuthLocator (#1303)

 ## Fixes
`RegistryAuthLocator` : Add additional validation for existing key `credsStore` but empty value

## Purpose
Define behaviour for RegistryAuthLocator on empty values in config

Offer: 
 - fail on empty value
 - ignore field with empty value & log -> return null;

## Background Context
Currently experienced problems using testcontainers, I found existing issue for my problem and decided to help. 
Potentially, put checks for all existing `keys` with empty strings

## References
ref: #921

core/src/main/java/org/testcontainers/utility/RegistryAuthLocator.java

@@ -183,6 +183,10 @@ private AuthConfig authConfigUsingStore(final JsonNode config, final String repo
         final JsonNode credsStoreNode = config.get("credsStore");
         if (credsStoreNode != null && !credsStoreNode.isMissingNode() && credsStoreNode.isTextual()) {
             final String credsStore = credsStoreNode.asText();
+            if (isBlank(credsStore)) {
+                log.warn("Docker auth config credsStore field will be ignored, because value is blank");
+                return null;
+            }
             return runCredentialProvider(reposName, credsStore);
         }
         return null;

core/src/test/java/org/testcontainers/utility/RegistryAuthLocatorTest.java

@@ -101,6 +101,16 @@ public void lookupAuthConfigWithCredentialsNotFound() throws URISyntaxException
             discoveredMessage);
     }
 
+    @Test
+    public void lookupAuthConfigWithCredStoreEmpty() throws URISyntaxException {
+        final RegistryAuthLocator authLocator = createTestAuthLocator("config-with-store-empty.json");
+
+        DockerImageName dockerImageName = new DockerImageName("registry2.example.com/org/repo");
+        final AuthConfig authConfig = authLocator.lookupAuthConfig(dockerImageName, new AuthConfig());
+
+        assertNull("CredStore field will be ignored, because value is blank", authConfig.getAuth());
+    }
+
     @NotNull
     private RegistryAuthLocator createTestAuthLocator(String configName) throws URISyntaxException {
         return createTestAuthLocator(configName, new HashMap<>());

core/src/test/resources/auth-config/config-with-store-empty.json

@@ -0,0 +1,9 @@
+{
+    "auths": {
+        "registry.example.com": {}
+    },
+    "HttpHeaders": {
+        "User-Agent": "Docker-Client/18.03.0-ce (darwin)"
+    },
+    "credsStore": ""
+}