Handle null Regex in the config as the empty regex. (#2150)

config/config.go

@@ -1017,6 +1017,9 @@ func (c *RelabelConfig) UnmarshalYAML(unmarshal func(interface{}) error) error {
 	if err := checkOverflow(c.XXX, "relabel_config"); err != nil {
 		return err
 	}
+	if c.Regex.Regexp == nil {
+		c.Regex = MustNewRegexp("")
+	}
 	if c.Modulus == 0 && c.Action == RelabelHashMod {
 		return fmt.Errorf("relabel configuration for hashmod requires non-zero modulus")
 	}

config/config_test.go

@@ -115,6 +115,12 @@ var expectedConf = &Config{
 					Regex:       DefaultRelabelConfig.Regex,
 					Replacement: "static",
 					Action:      RelabelReplace,
+				}, {
+					TargetLabel: "abc",
+					Separator:   ";",
+					Regex:       MustNewRegexp(""),
+					Replacement: "static",
+					Action:      RelabelReplace,
 				},
 			},
 		},

config/testdata/conf.good.yml

@@ -54,6 +54,9 @@ scrape_configs:
     target_label:  cde
   - replacement:   static
     target_label:  abc
+  - regex:
+    replacement:   static
+    target_label:  abc
 
   bearer_token_file: valid_token_file