tdf#159930 Treat special case with no integer digit

Number format with no integer digit, such as "+.##;-.##"
was placing string with the decimal separator.
This change insert string at the begining of the number string as
expected.
This format code is saved as "+#.##;-#.##" in ODF, but display is the
same; so no change is made on this side.

Add unit test for this format

Change-Id: I74fbe0e9a5303672ac7927d37922c06a762feba6
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/168577
Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Jenkins

svl/qa/unit/svl.cxx

@@ -1823,6 +1823,11 @@ void Test::testUserDefinedNumberFormats()
         sExpected = "Value= 12,345.00";
         checkPreviewString(aFormatter, sCode, 12345, LANGUAGE_ENGLISH_US, sExpected);
     }
+    {   // tdf#159930 no digit in integer part
+        sCode = "+.000;-.000";
+        sExpected = "+3.142"; // without the patch is would display "3+.142"
+        checkPreviewString(aFormatter, sCode, M_PI, LANGUAGE_ENGLISH_US, sExpected);
+    }
 }
 
 void Test::testNfEnglishKeywordsIntegrity()

svl/source/numbers/zformat.cxx

@@ -4682,6 +4682,12 @@ bool SvNumberformat::ImpNumberFillWithThousands( const NativeNumberWrapper& rNat
             aGrouping.reset();
             [[fallthrough]];
         case NF_SYMBOLTYPE_STRING:
+            if ( rInfo.nTypeArray[j] == NF_SYMBOLTYPE_STRING && nDigCnt == 0 )
+            {
+                // tdf#159930 no integer in format ".###"
+                k = 0; // insert string at the beginning
+            }
+            [[fallthrough]];
         case NF_SYMBOLTYPE_CURRENCY:
         case NF_SYMBOLTYPE_PERCENT:
             if ( rInfo.nTypeArray[j] != NF_SYMBOLTYPE_DECSEP || bAddDecSep )