Merge remote-tracking branch 'origin/topic/christian/gh-1982'

* origin/topic/christian/gh-1982:
  The is_num(), is_alpha(), and is_alnum() BiFs now return F on empty string
  Expand testcases around is_num(), is_alpha(), is_alnum(), is_ascii() BiFs
  Clarify is_ascii() BiF docstring re behavior on empty strings
  Fix minor indentation bugs in strings.bif

testing/btest/Baseline/bifs.is_ascii/out

@@ -1,3 +1,4 @@
 ### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
 F
 T
+T

testing/btest/Baseline/bifs.string_utils/out

@@ -17,14 +17,17 @@ Content checking
 ----------------
 is_num abc   : 0
 is_num 123   : 1
+is_num ''    : 0
 is_alpha ab  : 1
 is_alpha 1a  : 0
 is_alpha a1  : 0
+is_alpha ''  : 0
 is_alnum ab  : 1
 is_alnum 1a  : 1
 is_alnum a1  : 1
 is_alnum 12  : 1
 is_alnum ##12: 0
+is_alnum ''  : 0
 
 String counting (input str 'aabbaa')
 ------------------------------------

testing/btest/bifs/is_ascii.zeek

@@ -9,4 +9,5 @@ event zeek_init()
 
 	print is_ascii(a);
 	print is_ascii(b);
+	print is_ascii("");
 	}

testing/btest/bifs/string_utils.zeek

@@ -23,14 +23,17 @@ event zeek_init()
 	print "----------------";
 	print fmt("is_num abc   : %d", is_num("abc"));
 	print fmt("is_num 123   : %d", is_num("123"));
+	print fmt("is_num ''    : %d", is_num(""));
 	print fmt("is_alpha ab  : %d", is_alpha("ab"));
 	print fmt("is_alpha 1a  : %d", is_alpha("1a"));
 	print fmt("is_alpha a1  : %d", is_alpha("a1"));
+	print fmt("is_alpha ''  : %d", is_alpha(""));
 	print fmt("is_alnum ab  : %d", is_alnum("ab"));
 	print fmt("is_alnum 1a  : %d", is_alnum("1a"));
 	print fmt("is_alnum a1  : %d", is_alnum("a1"));
 	print fmt("is_alnum 12  : %d", is_alnum("12"));
 	print fmt("is_alnum ##12: %d", is_alnum("##12"));
+	print fmt("is_alnum ''  : %d", is_alnum(""));
 	print "";
 
 	print "String counting (input str 'aabbaa')";